Magnetic tape device and head tracking servo method

ABSTRACT

The magnetic tape device includes a TMR head (servo head); and a magnetic tape, in which a magnetic layer of the magnetic tape includes fatty acid ester, Ra measured regarding a surface of the magnetic layer is equal to or smaller than 2.0 nm, full widths at half maximum of spacing distribution measured by optical interferometry regarding a surface of the magnetic layer before and after performing a vacuum heating with respect to the magnetic tape are greater than 0 nm and equal to or smaller than 7.0 nm, a difference between spacings before and after the vacuum heating is greater than 0 nm and equal to or smaller than 8.0 nm, and ΔSFD (=SFD25° C.−SFD−190° C.) in a longitudinal direction of the magnetic tape is equal to or smaller than 0.50.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2017-065502 filed on Mar. 29, 2017. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape device and a headtracking servo method.

2. Description of the Related Art

Magnetic recording is used as a method of recording information on arecording medium. In the magnetic recording, information is recorded ona magnetic recording medium as a magnetized pattern. Informationrecorded on a magnetic recording medium is reproduced by reading amagnetic signal obtained from the magnetized pattern by a magnetic head.As a magnetic head used for such reproducing, various magnetic headshave been proposed (for example, see JP2004-185676A).

SUMMARY OF THE INVENTION

An increase in recording capacity (high capacity) of a magneticrecording medium is required in accordance with a great increase ininformation content in recent years. As means for realizing highcapacity, a technology of increasing a recording density of a magneticrecording medium is used. However, as the recording density increases, amagnetic signal (specifically, a leakage magnetic field) obtained from amagnetic layer tends to become weak. Accordingly, it is desired that ahigh-sensitivity magnetic head capable of reading a weak signal withexcellent sensitivity is used as a reproducing head. Regarding thesensitivity of the magnetic head, it is said that a magnetoresistive(MR) head using a magnetoresistance effect as an operating principle hasexcellent sensitivity, compared to an inductive head used in the relatedart.

As the MR head, an anisotropic magnetoresistive (AMR) head and a giantmagnetoresistive (GMR) head are known as disclosed in a paragraph 0003of JP2004-185676A. The GMR head is an MR head having excellentsensitivity than that of the AMR head. In addition, a tunnelmagnetoresistive (TMR) head disclosed in a paragraph 0004 and the likeof JP2004-185676A is an MR head having a high possibility of realizinghigher sensitivity.

Meanwhile, a recording and reproducing system of the magnetic recordingis broadly divided into a levitation type and a sliding type. A magneticrecording medium in which information is recorded by the magneticrecording is broadly divided into a magnetic disk and a magnetic tape.Hereinafter, a drive including a magnetic disk as a magnetic recordingmedium is referred to as a “magnetic disk device” and a drive includinga magnetic tape as a magnetic recording medium is referred to as a“magnetic tape device”.

The magnetic disk device is generally called a hard disk drive (HDD) anda levitation type recording and reproducing system is used. In themagnetic disk device, a shape of a surface of a magnetic head sliderfacing a magnetic disk and a head suspension assembly that supports themagnetic head slider are designed so that a predetermined intervalbetween a magnetic disk and a magnetic head can be maintained with airflow at the time of rotation of the magnetic disk. In such a magneticdisk device, information is recorded and reproduced in a state where themagnetic disk and the magnetic head do not come into contact with eachother. The recording and reproducing system described above is thelevitation type. On the other hand, a sliding type recording andreproducing system is used in the magnetic tape device. In the magnetictape device, a surface of a magnetic layer of a magnetic tape and amagnetic head come into contact with each other and slide on each other,at the time of the recording and reproducing information.

JP2004-185676A proposes usage of the TMR head as a reproducing head forreproducing information in the magnetic disk device. On the other hand,the usage of the TMR head as a reproducing head in the magnetic tapedevice is currently still in a stage where the future usage thereof isexpected, and the usage thereof is not yet practically realized.

However, in the magnetic tape, information is normally recorded on adata band of the magnetic tape. Accordingly, data tracks are formed inthe data band. As means for realizing high capacity of the magnetictape, a technology of disposing the larger amount of data tracks in awidth direction of the magnetic tape by narrowing the width of the datatrack to increase recording density is used. However, in a case wherethe width of the data track is narrowed and the recording and/orreproduction of information is performed by transporting the magnetictape in the magnetic tape device, it is difficult that a magnetic headproperly follows the data tracks in accordance with the position changeof the magnetic tape, and errors may easily occur at the time ofrecording and/or reproduction. Thus, as means for preventing occurrenceof such errors, a method of forming a servo pattern in the magneticlayer and performing head tracking servo has been recently proposed andpractically used. In a magnetic servo type head tracking servo amonghead tracking servos, a servo pattern is formed in a magnetic layer of amagnetic tape, and this servo pattern is read by a servo head to performhead tracking servo. The head tracking servo is to control a position ofa magnetic head in the magnetic tape device. The head tracking servo ismore specifically performed as follows.

First, a servo head reads a servo pattern to be formed in a magneticlayer (that is, reproduces a servo signal). A position of a magnetichead in a magnetic tape device is controlled in accordance with a valueobtained by reading the servo pattern. Accordingly, in a case oftransporting the magnetic tape in the magnetic tape device for recordingand/or reproducing information, it is possible to increase an accuracyof the magnetic head following the data track, even in a case where theposition of the magnetic tape is changed. For example, even in a casewhere the position of the magnetic tape is changed in the widthdirection with respect to the magnetic head, in a case of recordingand/or reproducing information by transporting the magnetic tape in themagnetic tape device, it is possible to control the position of themagnetic head of the magnetic tape in the width direction in themagnetic tape device, by performing the head tracking servo. By doingso, it is possible to properly record information in the magnetic tapeand/or properly reproduce information recorded on the magnetic tape inthe magnetic tape device.

The servo pattern is formed by magnetizing a specific position of themagnetic layer. A plurality of regions including a servo pattern(referred to as “servo bands”) are generally present in the magnetictape capable of performing the head tracking servo along a longitudinaldirection. A region interposed between two servo bands is referred to asa data band. The recording of information is performed on the data bandand a plurality of data tracks are formed in each data band along thelongitudinal direction. In order to realize high capacity of themagnetic tape, it is preferable that the larger number of the data bandswhich are regions where information is recorded are present in themagnetic layer. As means for that, a technology of increasing apercentage of the data bands occupying the magnetic layer by narrowingthe width of the servo band which is not a region in which informationis recorded is considered. In regards to this point, the inventors haveconsidered that, since a read track width of the servo pattern becomesnarrow, in a case where the width of the servo band becomes narrow, itis desired to use a magnetic head having high sensitivity as the servohead, in order to ensure reading accuracy of the servo pattern. As amagnetic head for this, the inventors focused on a TMR head which hasbeen proposed to be used as a reproducing head in the magnetic diskdevice in JP2004-185676A. As described above, the usage of the TMR headin the magnetic tape device is still in a stage where the future usethereof as a reproducing head for reproducing information is expected,and the usage of the TMR head as the servo head has not even proposedyet. However, the inventors have thought that, it is possible to dealwith realization of higher sensitivity of the future magnetic tape, in acase where the TMR head is used as the servo head in the magnetic tapedevice which performs the head tracking servo.

In addition, a signal-to-noise-ratio (SNR) at the time of reading theservo pattern tends to decrease in accordance with a decrease in readtrack width of the servo pattern. However, a decrease in SNR at the timeof reading the servo pattern causes a decrease in accuracy that themagnetic head follows the data track by the head tracking servo.

Therefore, an object of the invention is to provide a magnetic tapedevice in which a TMR head is mounted as a servo head and a servopattern written on a magnetic tape can be read at a high SNR.

As means for increasing the SNR at the time of reproducing informationrecorded on the magnetic tape, a method of increasing smoothness of asurface of a magnetic layer of a magnetic tape is used. This point isalso preferable for increasing the SNR in a case of reading a servopattern written in the magnetic tape. The inventors have made intensivestudies for further increasing the SNR in a case of reading a servopattern written in the magnetic tape, by using other methods, inaddition to the method of increasing smoothness of a surface of amagnetic layer of a magnetic tape.

Meanwhile, a magnetoresistance effect which is an operating principle ofthe MR head such as the TMR head is a phenomenon in which electricresistance changes depending on a change in magnetic field. The MR headdetects a change in leakage magnetic field generated from a magneticrecording medium as a change in resistance value (electric resistance)and reproduces information by converting the change in resistance valueinto a change in voltage. In a case where the TMR head is used as theservo head, the TMR head detects a change in leakage magnetic fieldgenerated from a magnetic layer in which the servo pattern is formed, asa change in resistance value (electric resistance) and reads the servopattern (reproduces a servo signal) by converting the change inresistance value into a change in voltage. It is said that a resistancevalue in the TMR head is generally high, as disclosed in a paragraph0007 of JP2004-185676A, but generation of a significant decrease inresistance value in the TMR head, while continuing the reading of aservo pattern with the TMR head, may cause a decrease in sensitivity ofthe TMR head, while continuing the head tracking servo. As a result, theaccuracy of head position controlling of the head tracking servo maydecrease, while continuing the head tracking servo.

During intensive studies for achieving the object described above, theinventors have found a phenomenon which was not known in the relatedart, in that, in a case of using the TMR head as a servo head in themagnetic tape device which performs the head tracking servo, asignificant decrease in resistance value (electric resistance) occurs inthe TMR head. A decrease in resistance value in the TMR head is adecrease in electric resistance measured by bringing an electricresistance measuring device into contact with a wiring connecting twoelectrodes configuring a tunnel magnetoresistance effect type elementincluded in the TMR head. The phenomenon in which this resistance valuesignificantly decreases is not observed in a case of using the TMR headin the magnetic disk device, nor in a case of using other MR heads suchas the GMR head in the magnetic disk device or the magnetic tape device.That is, occurrence of a significant decrease in resistance value in theTMR head in a case of using the TMR head was not even confirmed in therelated art. A difference in the recording and reproducing systembetween the magnetic disk device and the magnetic tape device,specifically, contact and non-contact between a magnetic recordingmedium and a magnetic head may be the reason why a significant decreasein resistance value in the TMR head occurred in the magnetic tape deviceis not observed in the magnetic disk device. In addition, the TMR headhas a special structure in which two electrodes are provided with aninsulating layer (tunnel barrier layer) interposed therebetween in adirection in which a magnetic tape is transported, which is not appliedto other MR heads which are currently practically used, and it isconsidered that this is the reason why a significant decrease inresistance value occurring in the TMR head is not observed in other MRheads. It is clear that, a significant decrease in resistance value inthe TMR head tends to more significantly occur in a magnetic tape devicein which a magnetic tape having high smoothness of a surface of amagnetic layer is mounted as the magnetic tape. With respect to this, asa result of more intensive studies after finding the phenomenondescribed above, the inventors have newly found that such a significantdecrease in resistance value can be prevented by using a magnetic tapedescribed below as the magnetic tape.

One aspect of the invention has been completed based on the findingdescribed above.

That is, according to one aspect of the invention, there is provided amagnetic tape device comprising: a magnetic tape; and a servo head, inwhich the servo head is a magnetic head (hereinafter, also referred toas a “TMR head”) including a tunnel magnetoresistance effect typeelement (hereinafter, also referred to as a “TMR element”) as a servopattern reading element, the magnetic tape includes a non-magneticsupport, and a magnetic layer including ferromagnetic powder, a bindingagent, and fatty acid ester on the non-magnetic support, the magneticlayer includes the servo pattern, a center line average surfaceroughness Ra measured regarding a surface of the magnetic layer(hereinafter, also referred to as a “magnetic layer surface roughnessRa”) is equal to or smaller than 2.0 nm, a full width at half maximum ofspacing distribution measured by optical interferometry regarding thesurface of the magnetic layer before performing a vacuum heating withrespect to the magnetic tape (hereinafter, also referred to as“FWHM_(before)”) is greater than 0 nm and equal to or smaller than 7.0nm, a full width at half maximum of spacing distribution measured byoptical interferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape(hereinafter, also referred to as “FWHM_(after)”) is greater than 0 nmand equal to or smaller than 7.0 nm, a difference (S_(after)−S_(before))between a spacing S_(after) measured by optical interferometry regardingthe surface of the magnetic layer after performing the vacuum heatingwith respect to the magnetic tape and a spacing S_(before) measured byoptical interferometry regarding the surface of the magnetic layerbefore performing the vacuum heating with respect to the magnetic tape(hereinafter, also simply referred to as a “difference(S_(after)−S_(before))”) is greater than 0 nm and equal to or smallerthan 8.0 nm, and ΔSFD in a longitudinal direction of the magnetic tapecalculated by Expression 1 (hereinafter, also simply referred to as“ΔSFD”), ΔSFD=SFD_(25° C.)−SFD_(−190 ° C.) . . . Expression 1, is equalto or smaller than 0.50. In Expression 1, the SFD_(25° C.) is aswitching field distribution SFD measured in a longitudinal direction ofthe magnetic tape at a temperature of 25° C., and the SFD_(−190° C.) isa switching field distribution SFD measured in a longitudinal directionof the magnetic tape at a temperature of −190° C.

According to another aspect of the invention, there is provided a headtracking servo method comprising: reading a servo pattern of a magneticlayer of a magnetic tape by a servo head in a magnetic tape device, inwhich the servo head is a magnetic head including a tunnelmagnetoresistance effect type element as a servo pattern readingelement, the magnetic tape includes a non-magnetic support, and amagnetic layer including ferromagnetic powder, a binding agent, andfatty acid ester on the non-magnetic support, the magnetic layerincludes the servo pattern, a center line average surface roughness Rameasured regarding a surface of the magnetic layer is equal to orsmaller than 2.0 nm, a full width at half maximum of spacingdistribution measured by optical interferometry regarding the surface ofthe magnetic layer before performing a vacuum heating with respect tothe magnetic tape is greater than 0 nm and equal to or smaller than 7.0nm, a full width at half maximum of spacing distribution measured byoptical interferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape isgreater than 0 nm and equal to or smaller than 7.0 nm, a difference(S_(after)−S_(before)) between a spacing S_(after) measured by opticalinterferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape and aspacing S_(before) measured by optical interferometry regarding thesurface of the magnetic layer before performing the vacuum heating withrespect to the magnetic tape is greater than 0 nm and equal to orsmaller than 8.0 nm, and ΔSFD in a longitudinal direction of themagnetic tape calculated by Expression 1 is equal to or smaller than0.50.

In the invention and the specification, the “vacuum heating” of themagnetic tape is performed by holding the magnetic tape in anenvironment of a pressure of 200 Pa to 0.01 MPa and at an atmospheretemperature of 70° C. to 90° C. for 24 hours.

In the invention and the specification, the spacing measured by opticalinterferometry regarding the surface of the magnetic layer of themagnetic tape is a value measured by the following method. In theinvention and the specification, the “surface of the magnetic layer” ofthe magnetic tape is identical to the surface of the magnetic tape onthe magnetic layer side.

In a state where the magnetic tape and a transparent plate-shaped member(for example, glass plate or the like) are overlapped onto each other sothat the surface of the magnetic layer of the magnetic tape faces thetransparent plate-shaped member, a pressing member is pressed againstthe side of the magnetic tape opposite to the magnetic layer side atpressure of 5.05×10⁴ N/m (0.5 atm). In this state, the surface of themagnetic layer of the magnetic tape is irradiated with light through thetransparent plate-shaped member (irradiation region: 150,000 to 200,000μm²), and a spacing (distance) between the surface of the magnetic layerof the magnetic tape and the surface of the transparent plate-shapedmember on the magnetic tape side is acquired based on intensity (forexample, contrast of interference fringe image) of interference lightgenerated due to a difference in a light path between reflected lightfrom the surface of the magnetic layer of the magnetic tape andreflected light from the surface of the transparent plate-shaped memberon the magnetic tape side. The light emitted here is not particularlylimited. In a case where the emitted light is light having an emissionwavelength over a comparatively wide wavelength range as white lightincluding light having a plurality of wavelengths, a member having afunction of selectively cutting light having a specific wavelength or awavelength other than wavelengths in a specific wavelength range, suchas an interference filter, is disposed between the transparentplate-shaped member and a light reception unit which receives reflectedlight, and light at some wavelengths or in some wavelength ranges of thereflected light is selectively incident to the light reception unit. Ina case where the light emitted is light (so-called monochromatic light)having a single luminescence peak, the member described above may not beused. The wavelength of light incident to the light reception unit canbe set to be 500 to 700 nm, for example. However, the wavelength oflight incident to the light reception unit is not limited to be in therange described above. In addition, the transparent plate-shaped membermay be a member having transparency through which emitted light passes,to the extent that the magnetic tape is irradiated with light throughthis member and interference light is obtained.

The measurement described above can be performed by using a commerciallyavailable tape spacing analyzer (TSA) such as Tape Spacing Analyzermanufactured by Micro Physics, Inc., for example. The spacingmeasurement of the examples was performed by using Tape Spacing Analyzermanufactured by Micro Physics, Inc.

In addition, the full width at half maximum of spacing distribution ofthe invention and the specification is a full width at half maximum(FWHM), in a case where the interference fringe image obtained by themeasurement of the spacing described above is divided into 300,000points, a spacing of each point (distance between the surface of themagnetic layer of the magnetic tape and the surface of the transparentplate-shaped member on the magnetic tape side) is acquired, this spacingis shown with a histogram, and this histogram is fit with Gaussiandistribution.

Further, the difference (S_(after)−S_(before)) is a value obtained bysubtracting a mode before the vacuum heating from a mode after thevacuum heating of the 300,000 points.

One aspect of the magnetic tape device and the head tracking servomethod is as follows.

In one aspect, the FWHM_(before) is 3.0 nm to 7.0 nm.

In one aspect, the FWHM_(after) is 3.0 nm to 7.0 nm.

In one aspect, the difference (S_(after)−S_(before)) is 2.0 nm to 8.0nm.

In one aspect, the center line average surface roughness Ra measuredregarding the surface of the magnetic layer is 1.2 nm to 2.0 nm.

In one aspect, the ΔSFD is 0.03 to 0.50.

In one aspect, the magnetic tape includes a non-magnetic layer includingnon-magnetic powder and a binding agent between the non-magnetic supportand the magnetic layer.

According to one aspect of the invention, it is possible to perform thereading at a high SNR, in a case of reading a servo pattern of themagnetic layer of the magnetic tape with the TMR head and preventoccurrence of a significant decrease in resistance value in the TMRhead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of disposition of data bands and servo bands.

FIG. 2 shows a servo pattern disposition example of a linear-tape-open(LTO) Ultrium format tape.

FIG. 3 is a schematic configuration diagram of a vibration impartingdevice used in examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape Device

One aspect of the invention relates to a magnetic tape device including:a magnetic tape; and a servo head, in which the servo head is a magnetichead including a tunnel magnetoresistance effect type element as a servopattern reading element, the magnetic tape includes a non-magneticsupport, and a magnetic layer including ferromagnetic powder, a bindingagent, and fatty acid ester on the non-magnetic support, the magneticlayer includes a servo pattern, a center line average surface roughnessRa measured regarding a surface of the magnetic layer (magnetic layersurface roughness Ra) is equal to or smaller than 2.0 nm, a full widthat half maximum of spacing distribution measured by opticalinterferometry regarding the surface of the magnetic layer beforeperforming a vacuum heating with respect to the magnetic tape(FWHM_(before)) is greater than 0 nm and equal to or smaller than 7.0nm, a full width at half maximum of spacing distribution measured byoptical interferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape(FWHM_(after)) is greater than 0 nm and equal to or smaller than 7.0 nm,a difference (S_(after)−S_(before)) between a spacing S_(after) measuredby optical interferometry regarding the surface of the magnetic layerafter performing the vacuum heating with respect to the magnetic tapeand a spacing S_(before) measured by optical interferometry regardingthe surface of the magnetic layer before performing the vacuum heatingwith respect to the magnetic tape is greater than 0 nm and equal to orsmaller than 8.0 nm, and ΔSFD in a longitudinal direction of themagnetic tape calculated by Expression 1 is equal to or smaller than0.50.

The inventors have thought that the magnetic layer surface roughness Raand the ΔSFD set to be in the ranges described above contribute to thereading of a servo pattern written in the magnetic layer of the magnetictape in the magnetic tape device at a high SNR, and the FWHM_(before),the FWHM_(after), and the difference (S_(after)−S_(before)) set to be inthe range described above contributes to the prevention of a significantdecrease in resistance value in the TMR head.

The magnetic layer surface roughness Ra equal to or smaller than 2.0 nmcan contribute to a decrease in spacing loss causing a decrease in SNR.In addition, the ΔSFD equal to or smaller than 0.50 also contribute toimprovement of the SNR. It is thought that the ΔSFD is a value which maybe an index for a state of ferromagnetic powder present in the magneticlayer. It is surmised that, a state in which the ΔSFD is equal to orsmaller than 0.50 is a state in which particles of ferromagnetic powderis suitably aligned and present in the magnetic layer, and such a statecontributes to the reading of a servo pattern written in the magneticlayer at a high SNR.

The above description is a surmise of the inventors regarding thereading of a servo pattern written in the magnetic layer of the magnetictape at a high SNR, in the magnetic tape device. The inventors havethought regarding the usage of the TMR head by preventing the occurrenceof a significant decrease in resistance value, in the magnetic tape, asfollows.

In the magnetic tape device, in a case of using a magnetic tape of therelated art, in a case of using a TMR head as a servo head forperforming head tracking servo at the time of recording and/orreproducing information, a phenomenon in which a resistance value(electric resistance) significantly decreases in the TMR head occurs.This phenomenon is a phenomenon that is newly found by the inventors.The inventors have considered the reason for the occurrence of such aphenomenon is as follows.

The TMR head is a magnetic head using a tunnel magnetoresistance effectand includes two electrodes with an insulating layer (tunnel barrierlayer) interposed therebetween. The tunnel barrier layer positionedbetween the two electrodes is an insulating layer, and thus, even in acase where a voltage is applied between the two electrodes, in general,a current does not flow or does not substantially flow between theelectrodes. However, a current (tunnel current) flows by a tunnel effectdepending on a direction of a magnetic field of a free layer affected bya leakage magnetic field from the magnetic tape, and a change in amountof a tunnel current flow is detected as a change in resistance value bythe tunnel magnetoresistance effect. By converting the change inresistance value into a change in voltage, a servo pattern formed in themagnetic tape can be read (a servo signal can be reproduced).

Examples of a structure of the MR head include a current-in-plane (CIP)structure and a current-perpendicular-to-plane (CPP) structure, and theTMR head is a magnetic head having a CPP structure. In the MR headhaving a CPP structure, a current flows in a direction perpendicular toa film surface of an MR element, that is, a direction in which themagnetic tape is transported, in a case of reading a servo patternformed in the magnetic tape. With respect to this, other MR heads, forexample, a spin valve type GMR head which is widely used in recent yearsamong the GMR heads has a CIP structure. In the MR head having a CIPstructure, a current flows in a direction in a film plane of an MRelement, that is, a direction perpendicular to a direction in which themagnetic tape is transported, in a case of reading a servo patternformed in the magnetic tape.

As described above, the TMR head has a special structure which is notapplied to other MR heads which are currently practically used.Accordingly, in a case where short circuit (bypass due to damage) occurseven at one portion between the two electrodes, the resistance valuesignificantly decreases. A significant decrease in resistance value in acase of the short circuit occurred even at one portion between the twoelectrodes as described above is a phenomenon which does not occur inother MR heads. In the magnetic disk device using a levitation typerecording and reproducing system, a magnetic disk and a magnetic head donot come into contact with each other, and thus, damage causing shortcircuit hardly occurs. On the other hand, in the magnetic tape deviceusing a sliding type recording and reproducing system, the magnetic tapeand the servo head come into contact with each other and slide on eachother, in a case of reading a servo pattern by the servo head.Accordingly, in a case where any measures are not prepared, the TMR headis damaged due to the sliding between the TMR head and the magnetictape, and thus, short circuit easily occurs. The inventors have assumedthat this is the reason why a decrease in resistance value of the TMRhead significantly occurs, in a case of using the TMR head as the servohead in the magnetic tape device. In addition, it is thought that, in acase where the smoothness of the surface of the magnetic layer of themagnetic tape increases, a contact area (so-called real contact area)between the surface of the magnetic layer and the servo head increases.It is thought that the servo head which is more easily damaged at thetime of sliding on the magnetic tape due to an increase in contact area,is a reason a decrease in resistance value in the TMR head which tendsto be significant, in the magnetic tape device in which the magnetictape having high smoothness of the surface of the magnetic layer ismounted.

With respect to this, as a result of intensive studies of the inventors,the inventors have newly found that it is possible to prevent aphenomenon in which a decrease in resistance value of the TMR headsignificantly occurs, in a case of using the TMR head as the servo headin the magnetic tape device, by using the magnetic tape in which theFWHM_(before), the FWHM_(after), and the difference(S_(after)−S_(before)) are respectively in the ranges described above.The surmise of the inventors regarding this point is as described in thefollowing (1) and (2).

(1) A portion (projection) which mainly comes into contact (so-calledreal contact) with the servo head in a case where the magnetic tape andthe servo head slide on each other, and a portion (hereinafter, referredto as a “base portion”) having a height lower than that of the portiondescribed above are normally present on the surface of the magneticlayer. The inventors have thought that the spacing described above is avalue which is an index of a distance between the servo head and thebase portion in a case where the magnetic tape and the servo head slideon each other. However, it is thought that, in a case where a lubricantincluded on the magnetic layer forms a liquid film on the surface of themagnetic layer, the liquid film is present between the base portion andthe servo head, and thus, the spacing is narrowed by the thickness ofthe liquid film.

Meanwhile, the lubricant is generally divided broadly into a liquidlubricant and a boundary lubricant. Fatty acid ester included in themagnetic layer of the magnetic tape is known as a component which canfunction as a liquid lubricant. It is considered that a liquid lubricantcan protect the surface of the magnetic layer by forming a liquid filmon the surface of the magnetic layer. The inventors have thought thatthe presence of the liquid film of fatty acid ester on the surface ofthe magnetic layer contributes to the smooth sliding (improvement ofsliding properties) between the magnetic tape and the servo head (TMRhead). However, an excessive amount of fatty acid ester present on thesurface of the magnetic layer causes sticking due to the formation of ameniscus (liquid crosslinking) between the surface of the magnetic layerand the servo head due to fatty acid ester, thereby decreasing slidingproperties.

In regards to this point, the inventors focused on the idea that fattyacid ester is a component having properties of volatilizing by vacuumheating, and the difference (S_(after)−S_(before)) of a spacing betweena state after the vacuum heating (state in which a liquid film of fattyacid ester is volatilized and removed) and a state before the vacuumheating (state in which the liquid film of fatty acid ester is present)was used as an index of a thickness of the liquid film formed of fattyacid ester on the surface of the magnetic layer. The inventors havesurmised that the presence of the liquid film of fatty acid ester on thesurface of the magnetic layer, so that the value of the difference isgreater than 0 nm and equal to or smaller than 8.0 nm, causes theimprovement of sliding properties between the servo head (TMR head) andthe magnetic tape while preventing sticking.

(2) A smaller value of the full width at half maximum of spacingdistribution means that a variation in the values of the spacingmeasured on each part of the surface of the magnetic layer is small. Asa result of the intensive studies, the inventors found that it iseffective to increase uniformity of a contact state between the surfaceof the magnetic layer and the servo head by increasing uniformity of aheight of projection present on the surface of the magnetic layer andincreasing uniformity of a thickness of a liquid film of fatty acidester, in order to realize smooth sliding between the magnetic tape andthe servo head.

In regards to this point, it is considered that the reason for thevariation in values of the spacing is a variation in height of theprojection of the surface of the magnetic layer and a variation in thethickness of the liquid film fatty acid ester. The inventors havesurmised that the full width at half maximum of the spacing distributionFWHM_(before) measured before the vacuum heating, that is, in a statewhere the liquid film of fatty acid ester is present on the surface ofthe magnetic layer, becomes great, as the variation in height of theprojection and the variation in the thickness of the liquid film offatty acid ester are great. Particularly, the spacing distributionFWHM_(before) is greatly affected by the variation in the thickness ofthe liquid film of fatty acid ester. In contrast, the inventors havesurmised that the full width at half maximum of the spacing distributionFWHM_(after) measured after the vacuum heating, that is, in a statewhere the liquid film of fatty acid ester is removed from the surface ofthe magnetic layer, becomes great, as the variation in height of theprojection is great. That is, the inventors have surmised that smallfull widths at half maximum of spacing distributions FWHM_(before) andFWHM_(after) mean a small variation in the thickness of the liquid filmof fatty acid ester on the surface of the magnetic layer and a smallvariation in height of the projection. It is thought that an increase inuniformity of the height of the projection and the thickness of theliquid film of fatty acid ester so that the full widths at half maximumof the spacing distribution FWHM_(before) and FWHM_(after) are greaterthan 0 nm and equal to or smaller than 7.0 nm contributes to smoothsliding between the magnetic tape and the TMR head. As a result, theinventors have surmised that it is possible to prevent occurrence ofshort circuit due to damage on the TMR head due to the sliding on themagnetic tape having the magnetic layer surface roughness Ra of 2.0 nmand excellent smoothness of the surface of the magnetic layer.

However, the above descriptions are merely a surmise of the inventorsand the invention is not limited thereto.

Hereinafter, the magnetic tape device will be described morespecifically. A “decrease in resistance value of the TMR head” describedbelow is a significant decrease in resistance value of the TMR headoccurring in a case of reading a servo pattern by using the TMR head asthe servo head, unless otherwise noted.

Magnetic Tape

Magnetic Layer Surface Roughness Ra

The center line average surface roughness Ra measured regarding thesurface of the magnetic layer of the magnetic tape (magnetic layersurface roughness Ra) is equal to or smaller than 2.0 nm. This point cancontribute to the reading of the servo pattern a high SNR in themagnetic tape device. From a viewpoint of further increasing the SNR,the magnetic layer surface roughness Ra is preferably equal to orsmaller than 1.9 nm, more preferably equal to or smaller than 1.8 nm,even more preferably equal to or smaller than 1.7 nm, still preferablyequal to or smaller than 1.6 nm, and still more preferably equal to orsmaller than 1.5 nm. In addition, the magnetic layer surface roughnessRa can be, for example, equal to or greater than 1.0 nm or equal to orgreater than 1.2 nm. However, from a viewpoint of increasing the SNR, alow magnetic layer surface roughness Ra is preferable, and thus, themagnetic layer surface roughness Ra may be lower than the lower limitexemplified above.

The center line average surface roughness Ra measured regarding thesurface of the magnetic layer of the magnetic tape in the invention andthe specification is a value measured with an atomic force microscope(AFM) in a region having an area of 40 μm×40 μm of the surface of themagnetic layer. As an example of the measurement conditions, thefollowing measurement conditions can be used. The magnetic layer surfaceroughness Ra shown in examples which will be described later is a valueobtained by the measurement under the following measurement conditions.

The measurement is performed regarding the region of 40 μm×40 μm of thearea of the surface of the magnetic layer of the magnetic tape with anAFM (Nanoscope 4 manufactured by Veeco Instruments, Inc.) in a tappingmode. RTESP-300 manufactured by BRUKER is used as a probe, a scan speed(probe movement speed) is set as 40 μm/sec, and a resolution is set as512 pixel×512 pixel.

The magnetic layer surface roughness Ra can be controlled by awell-known method. For example, the magnetic layer surface roughness Racan be changed in accordance with the size of various powders includedin the magnetic layer or manufacturing conditions of the magnetic tape.Thus, by adjusting one or more of these, it is possible to obtain themagnetic tape having the magnetic layer surface roughness Ra equal to orsmaller than 2.0 nm.

Full Width at Half Maximum of Spacing Distribution FWHM_(before) andFWHM_(after)

Both of the full width at half maximum of spacing distributionFWHM_(before) before the vacuum heating and the full width at halfmaximum of spacing distribution FWHM_(after) after the vacuum heatingwhich are measured in the magnetic tape are greater than 0 nm and equalto or smaller than 7.0 nm. The inventors have surmised that this pointcontributes to the prevention of a decrease in resistance value of theTMR head. From a viewpoint of further preventing a decrease inresistance value of the TMR head, the FWHM_(before) and the FWHM_(after)are preferably equal to or smaller than 6.5 nm, more preferably equal toor smaller than 6.0 nm, even more preferably equal to or smaller than5.5 nm, still more preferably equal to or smaller than 5.0 nm, and stilleven more preferably equal to or smaller than 4.5 nm. The FWHM_(before)and the FWHM_(after) can be, for example, equal to or greater than 0.5nm, equal to or greater than 1.0 nm, equal to or greater than 2.0 nm, orequal to or greater than 3.0 nm. Meanwhile, from a viewpoint ofpreventing a decrease in resistance value of the TMR head, it ispreferable that the values thereof are small, and therefore, the valuesthereof may be smaller than the exemplified values.

The full width at half maximum of spacing distribution FWHM_(before)before the vacuum heating can be decreased mainly by decreasing thevariation in the thickness of the liquid film of fatty acid ester. Anexample of a specific method will be described later. Meanwhile, thefull width at half maximum of spacing distribution FWHM_(after) afterthe vacuum heating can be decreased by decreasing the variation inheight of the projection of the surface of the magnetic layer. In orderto realize the decrease described above, it is preferable that apresence state of the powder component included in the magnetic layer,for example, non-magnetic filler, which will be described laterspecifically, in the magnetic layer is controlled. An example of aspecific method will be described later.

Difference (S_(after)−S_(before))

The difference (S_(after)−S_(before)) of the spacings before and afterthe vacuum heating measured in the magnetic tape is greater than 0 nmand equal to or smaller than 8.0 nm. The inventors have surmised thatthis point also contributes to the prevention of a decrease inresistance value of the TMR head. From a viewpoint of further preventinga decrease in resistance value of the TMR head, the difference(S_(after)−S_(before)) is preferably equal to or greater than 0.1 nm,more preferably equal to or greater than 1.0 nm, even more preferablyequal to or greater than 1.5 nm, still more preferably equal to orgreater than 2.0 nm, and still even more preferably equal to or greaterthan 2.5 nm. Meanwhile, from a viewpoint of further preventing adecrease in resistance value of the TMR head, the difference(S_(after)−S_(before)) is preferably equal to or smaller than 7.5 nm,more preferably equal to or smaller than 7.0 nm, even more preferablyequal to or smaller than 6.5 nm, still preferably equal to or smallerthan 6.0 nm, still more preferably equal to or smaller than 5.5 nm,still even more preferably equal to or smaller than 5.0 nm, stillfurthermore preferably equal to or smaller than 4.5 nm, and still evenfurthermore preferably equal to or smaller than 4.0 nm. The difference(S_(after)−S_(before)) can be controlled by the amount of fatty acidester added to a magnetic layer forming composition. In addition,regarding the magnetic tape including a non-magnetic layer between thenon-magnetic support and the magnetic layer, the difference(S_(after)−S_(before)) can also be controlled by the amount of fattyacid ester added to a non-magnetic layer forming composition. This isbecause that the non-magnetic layer can play a role of holding alubricant and supplying the lubricant to the magnetic layer, and fattyacid ester included in the non-magnetic layer may be moved to themagnetic layer and present in the surface of the magnetic layer.

ΔSFD

In the magnetic tape, the ΔSFD in a longitudinal direction of themagnetic tape calculated by Expression 1 is equal to or smaller than0.50. It is thought that the ΔSFD is a value which may be an indexshowing a state of ferromagnetic powder present in the magnetic layer.Specifically, it is thought that, as a value of the ΔSFD is small,particles of the ferromagnetic powder are aligned by strong interaction.It is surmised that, a state where the ΔSFD is equal to or smaller than0.50 is a state where particles of the ferromagnetic powder are suitablyaligned and present in the magnetic layer, and such a state contributesto an increase in SNR at the time of reading a servo pattern written inthe magnetic layer of the magnetic tape by the TMR head. From aviewpoint of further increasing the SNR, the ΔSFD is preferably equal toor smaller than 0.48, more preferably equal to or smaller than 0.45,even more preferably equal to or smaller than 0.40, still morepreferably equal to or smaller than 0.35, and still even more preferablyequal to or smaller than 0.30. In addition, from a viewpoint offurthermore increasing the SNR, the ΔSFD is preferably equal to orgreater than 0.03, more preferably equal to or greater than 0.05, andeven more preferably equal to or greater than 0.10.

The SFD in a longitudinal direction of the magnetic tape can be measuredby using a well-known magnetic properties measurement device such as anoscillation sample type magnetic-flux meter. The same applies to themeasurement of the SFD of the ferromagnetic powder. In addition, ameasurement temperature of the SFD can be adjusted by setting themeasurement device.

According to the studies of the inventors, the ΔSFD calculated byExpression 1 can be controlled by a preparation method of the magnetictape, and mainly the following tendencies were seen: (A) the valuedecreases, as dispersibility of ferromagnetic powder in the magneticlayer increases; (B) the value decreases, as ferromagnetic powder havingsmall temperature dependency of SFD is used; and (C) the valuedecreases, as the particles of the ferromagnetic powder are aligned in alongitudinal direction of the magnetic layer (as a degree of orientationin a longitudinal direction increases), and the value increases, as adegree of orientation in a longitudinal direction decreases.

For example, regarding (A), dispersion conditions are reinforced (anincrease in dispersion time, a decrease in diameter and/or an increasein degree of filling of dispersion beads used in the dispersion, and thelike), and a dispersing agent is used. As a dispersing agent, awell-known dispersing agent or a commercially available dispersing agentcan be used.

Meanwhile, regarding (B), as an example, ferromagnetic powder in which adifference ΔSFD_(powder) between SFD of the ferromagnetic powdermeasured at a temperature of 100° C. and SFD thereof measured at atemperature of 25° C., which are calculated by Expression 2 is 0.05 to1.50, can be used, for example. However, even in a case where thedifference ΔSFD_(powder) is not in the range described above, it ispossible to control the ΔSFD of the magnetic tape calculated byExpression 1 to be equal to or smaller than 0.50 by other controllingmethods.

ΔSFD_(powder)=SFD_(powder100° C.)−SFD_(powder25° C.)   Expression 2

(In Expression 2, the SFD_(powder100° C.) is a switching fielddistribution SFD of ferromagnetic powder measured at a temperature of100° C., and the SFD_(powder25° C.) is a switching field distributionSFD of ferromagnetic powder measured at a temperature of 25° C.)

Regarding (C), the ΔSFD tends to decrease by performing the orientationprocess of the magnetic layer as longitudinal orientation. The ΔSFDtends to increase by performing the orientation process of the magneticlayer as homeotropic alignment or setting non-orientation withoutperforming the orientation process.

Accordingly, for example, it is possible to obtain a magnetic tape inwhich the ΔSFD calculated by Expression 1 is equal to or smaller than0.50, by respectively controlling one of the methods (A) to (C) or acombination of two or more arbitrary methods.

Next, the magnetic layer and the like included in the magnetic tape willbe described more specifically.

Magnetic Layer

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic tape. Fromthis viewpoint, ferromagnetic powder having an average particle sizeequal to or smaller than 50 nm is preferably used as the ferromagneticpowder. Meanwhile, the average particle size of the ferromagnetic powderis preferably equal to or greater than 10 nm, from a viewpoint ofstability of magnetization.

In one aspect, it is preferable to use ferromagnetic powder in which thedifference ΔSFD_(powder) between the SFD measured at a temperature of100° C. and the SFD measured at a temperature of 25° C., which arecalculated by Expression 2 is in the range described above.

As a preferred specific example of the ferromagnetic powder,ferromagnetic hexagonal ferrite powder can be used. An average particlesize of the ferromagnetic hexagonal ferrite powder is preferably 10 nmto 50 nm and more preferably 20 nm to 50 nm, from a viewpoint ofimprovement of recording density and stability of magnetization. Fordetails of the ferromagnetic hexagonal ferrite powder, descriptionsdisclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134to 0136 of JP2011-216149A, and paragraphs 0013 to 0030 of JP2012-204726Acan be referred to, for example.

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. An average particle size ofthe ferromagnetic metal powder is preferably 10 nm to 50 nm and morepreferably 20 nm to 50 nm, from a viewpoint of improvement of recordingdensity and stability of magnetization. For details of the ferromagneticmetal powder, descriptions disclosed in paragraphs 0137 to 0141 ofJP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can bereferred to, for example.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on printing paperso that the total magnification of 500,000 to obtain an image ofparticles configuring the powder. A target particle is selected from theobtained image of particles, an outline of the particle is traced with adigitizer, and a size of the particle (primary particle) is measured.The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesarbitrarily extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of the plurality ofparticles not only includes an aspect in which particles configuring theaggregate directly come into contact with each other, and also includesan aspect in which a binding agent or an additive which will bedescribed later is interposed between the particles. A term “particles”is also used for describing the powder.

As a method of collecting a sample powder from the magnetic tape inorder to measure the particle size, a method disclosed in a paragraph of0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter, and an average plate ratio is an arithmeticalmean of (maximum long diameter/thickness or height). In a case of thedefinition (3), the average particle size is an average diameter (alsoreferred to as an average particle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50 to 90 mass % and more preferably 60 to90 mass %. The components other than the ferromagnetic powder of themagnetic layer are at least a binding agent and fatty acid ester, andone or more kinds of additives may be arbitrarily included. A highfilling percentage of the ferromagnetic powder in the magnetic layer ispreferable from a viewpoint of improvement recording density.

Binding Agent

The magnetic tape is a coating type magnetic tape, and the magneticlayer includes a binding agent together with the ferromagnetic powder.As the binding agent, one or more kinds of resin is used. The resin maybe a homopolymer or a copolymer. As the binding agent, various resinsnormally used as a binding agent of the coating type magnetic recordingmedium can be used. For example, as the binding agent, a resin selectedfrom a polyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, an acrylic resin obtained by copolymerizing styrene,acrylonitrile, or methyl methacrylate, a cellulose resin such asnitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylalresin such as polyvinyl acetal or polyvinyl butyral can be used alone ora plurality of resins can be mixed with each other to be used. Amongthese, a polyurethane resin, an acrylic resin, a cellulose resin, and avinyl chloride resin are preferable. These resins can be used as thebinding agent even in the non-magnetic layer and/or a back coating layerwhich will be described later. For the binding agent described above,description disclosed in paragraphs 0028 to 0031 of JP2010-24113A can bereferred to. In addition, the binding agent may be a radiation curableresin such as an electron beam-curable resin. For the radiation curableresin, descriptions disclosed in paragraphs 0044 and 0045 ofJP2011-048878A can be referred to.

An average molecular weight of the resin used as the binding agent canbe, for example, 10,000 to 200,000 as a weight-average molecular weight.The weight-average molecular weight of the invention and thespecification is a value obtained by performing polystyrene conversionof a value measured by gel permeation chromatography (GPC). As themeasurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC Device: HLC-8120 (Manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In addition, a curing agent can also be used together with the bindingagent. As the curing agent, in one aspect, a thermosetting compoundwhich is a compound in which a curing reaction (crosslinking reaction)proceeds due to heating can be used, and in another aspect, aphotocurable compound in which a curing reaction (crosslinking reaction)proceeds due to light irradiation can be used. At least a part of thecuring agent is included in the magnetic layer in a state of beingreacted (crosslinked) with other components such as the binding agent,by proceeding the curing reaction in the magnetic layer forming step.The preferred curing agent is a thermosetting compound, polyisocyanateis suitable. For details of the polyisocyanate, descriptions disclosedin paragraphs 0124 and 0125 of JP2011-216149A can be referred to, forexample. The amount of the curing agent can be, for example, 0 to 80.0parts by mass with respect to 100.0 parts by mass of the binding agentin the magnetic layer forming composition, and is preferably 50.0 to80.0 parts by mass, from a viewpoint of improvement of strength of eachlayer such as the magnetic layer.

Fatty Acid Ester

The magnetic tape includes fatty acid ester in the magnetic layer. Thefatty acid ester may be included alone as one type or two or more typesthereof may be included. Examples of fatty acid ester include esters oflauric acid, myristic acid, palmitic acid, stearic acid, oleic acid,linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidicacid. Specific examples thereof include butyl myristate, butylpalmitate, butyl stearate (butyl stearate), neopentyl glycol dioleate,sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyloleate, isocetyl stearate, isotridecyl stearate, octyl stearate,isooctyl stearate, amyl stearate, and butoxyethyl stearate.

The content of fatty acid ester, as the content of the magnetic layerforming composition, is, for example, 0.1 to 10.0 parts by mass and ispreferably 1.0 to 7.0 parts by mass with respect to 100.0 parts by massof ferromagnetic powder. In a case of using two or more kinds ofdifferent fatty acid esters as the fatty acid ester, the content thereofis the total content thereof. In the invention and the specification,the same applies to content of other components, unless otherwise noted.In addition, in the invention and the specification, a given componentmay be used alone or used in combination of two or more kinds thereof,unless otherwise noted.

In a case where the magnetic tape includes a non-magnetic layer betweenthe non-magnetic support and the magnetic layer, the content of fattyacid ester in a non-magnetic layer forming composition is, for example,0 to 10.0 parts by mass and is preferably 0.1 to 8.0 parts by mass withrespect to 100.0 parts by mass of non-magnetic powder.

Other Lubricants

The magnetic tape includes fatty acid ester which is one kind oflubricants at least in the magnetic layer. The lubricants other thanfatty acid ester may be arbitrarily included in the magnetic layerand/or the non-magnetic layer. As described above, the lubricantincluded in the non-magnetic layer may be moved to the magnetic layerand present in the surface of the magnetic layer. As the lubricant whichmay be arbitrarily included, fatty acid can be used. In addition, fattyacid amide and the like can also be used. Fatty acid ester is known as acomponent which can function as a liquid lubricant, whereas fatty acidand fatty acid amide are known as a component which can function as aboundary lubricant. It is considered that the boundary lubricant is alubricant which can be adsorbed to a surface of powder (for example,ferromagnetic powder) and form a rigid lubricant film to decreasecontact friction.

Examples of fatty acid include lauric acid, myristic acid, palmiticacid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenicacid, erucic acid, and elaidic acid, and stearic acid, myristic acid,and palmitic acid are preferable, and stearic acid is more preferable.Fatty acid may be included in the magnetic layer in a state of salt suchas metal salt.

As fatty acid amide, amide of various fatty acid described above isused, and examples thereof include lauric acid amide, myristic acidamide, palmitic acid amide, and stearic acid amide.

Regarding fatty acid and a derivative of fatty acid (amide and ester), apart derived from fatty acid of the fatty acid derivative preferably hasa structure which is the same as or similar to that of fatty acid usedin combination. As an example, in a case of using stearic acid as fattyacid, it is preferable to use stearic acid ester and/or stearic acidamide.

The content of fatty acid in the magnetic layer forming composition is,for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts bymass, and more preferably 1.0 to 7.0 parts by mass, with respect to100.0 parts by mass of the ferromagnetic powder. The content of fattyacid amide in the magnetic layer forming composition is, for example, 0to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, and morepreferably 0 to 1.0 part by mass with respect to 100.0 parts by mass ofthe ferromagnetic powder.

In a case where the magnetic tape includes a non-magnetic layer betweenthe non-magnetic support and the magnetic layer, the content of fattyacid in the non-magnetic layer forming composition is, for example, 0 to10.0 parts by mass, preferably 1.0 to 10.0 parts by mass, and morepreferably 1.0 to 7.0 parts by mass with respect to 100.0 parts by massof the non-magnetic powder. The content of fatty acid amide in thenon-magnetic layer forming composition is, for example, 0 to 3.0 partsby mass and preferably 0 to 1.0 part by mass with respect to 100.0 partsby mass of the non-magnetic powder.

Other Components

The magnetic layer may include one or more kinds of additives, ifnecessary, together with the various components described above. As theadditives, a commercially available product can be suitably selected andused according to the desired properties. Alternatively, a compoundsynthesized by a well-known method can be used as the additives. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive which can be included in the magneticlayer include a non-magnetic filler, a dispersing agent, a dispersingassistant, an antibacterial agent, an antistatic agent, and anantioxidant. The non-magnetic filler is identical to the non-magneticpowder. As the non-magnetic filler, a non-magnetic filler (hereinafter,referred to as a “projection formation agent”) which can function as aprojection formation agent which forms projections suitably protrudedfrom the surface of the magnetic layer, and a non-magnetic filler(hereinafter, referred to as an “abrasive”) which can function as anabrasive can be used.

Non-Magnetic Filler

As the projection formation agent which is one aspect of thenon-magnetic filler, various non-magnetic powders normally used as aprojection formation agent can be used. These may be inorganicsubstances or organic substances. In one aspect, from a viewpoint ofhomogenization of friction properties, particle size distribution of theprojection formation agent is not polydispersion having a plurality ofpeaks in the distribution and is preferably monodisperse showing asingle peak. From a viewpoint of availability of monodisperse particles,the projection formation agent is preferably powder of inorganicsubstances (inorganic powder). Examples of the inorganic powder includepowder of inorganic oxide such as metal oxide, metal carbonate, metalsulfate, metal nitride, metal carbide, and metal sulfide, and powder ofinorganic oxide is preferable. The projection formation agent is morepreferably colloidal particles and even more preferably inorganic oxidecolloidal particles. In addition, from a viewpoint of availability ofmonodisperse particles, the inorganic oxide configuring the inorganicoxide colloidal particles are preferably silicon dioxide (silica). Theinorganic oxide colloidal particles are more preferably colloidal silica(silica colloidal particles). In the invention and the specification,the “colloidal particles” are particles which are not precipitated anddispersed to generate a colloidal dispersion, in a case where 1 g of theparticles is added to 100 mL of at least one organic solvent of at leastmethyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or amixed solvent including two or more kinds of the solvent described aboveat an arbitrary mixing ratio. The average particle size of the colloidalparticles is a value obtained by a method disclosed in a paragraph 0015of JP2011-048878A as a measurement method of an average particlediameter. In addition, in another aspect, the projection formation agentis preferably carbon black.

An average particle size of the projection formation agent is, forexample, 30 to 300 nm and is preferably 40 to 200 nm.

The abrasive which is another aspect of the non-magnetic filler ispreferably non-magnetic powder having Mohs hardness exceeding 8 and morepreferably non-magnetic powder having Mohs hardness equal to or greaterthan 9. A maximum value of Mohs hardness is 10 of diamond. Specifically,powders of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂,TiC, chromium oxide (Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), ironoxide, diamond, and the like can be used, and among these, aluminapowder such as α-alumina and silicon carbide powder are preferable. Inaddition, regarding the particle size of the abrasive, a specificsurface area which is an index for the particle size is, for example,equal to or greater than 14 m²/g, and is preferably 16 m²/g and morepreferably 18 m²/g. Further, the specific surface area of the abrasivecan be, for example, equal to or smaller than 40 m²/g. The specificsurface area is a value obtained by a nitrogen adsorption method (alsoreferred to as a Brunauer-Emmett-Teller (BET) 1 point method), and is avalue measured regarding primary particles. Hereinafter, the specificsurface area obtained by such a method is also referred to as a BETspecific surface area.

In addition, from a viewpoint that the projection formation agent andthe abrasive can exhibit the functions thereof in more excellent manner,the content of the projection formation agent of the magnetic layer ispreferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 partsby mass with respect to 100.0 parts by mass of the ferromagnetic powder.Meanwhile, the content of the magnetic layer is preferably 1.0 to 20.0parts by mass, more preferably 3.0 to 15.0 parts by mass, and even morepreferably 4.0 to 10.0 parts by mass with respect to 100.0 parts by massof the ferromagnetic powder.

As an example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive of the magnetic layer formingcomposition. It is preferable to improve dispersibility of the magneticlayer forming composition of the non-magnetic filler such as anabrasive, in order to decrease the magnetic layer surface roughness Ra.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape mayinclude a magnetic layer directly on a non-magnetic support, or mayinclude a non-magnetic layer including non-magnetic powder and a bindingagent between the non-magnetic support and the magnetic layer. Thenon-magnetic powder used in the non-magnetic layer may be powder ofinorganic substances or powder of organic substances. In addition,carbon black and the like can be used. Examples of the inorganicsubstances include metal, metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, and metal sulfide. These non-magneticpowder can be purchased as a commercially available product or can bemanufactured by a well-known method. For details thereof, descriptionsdisclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referredto. For carbon black which can be used in the non-magnetic layer,descriptions disclosed in paragraphs 0040 and 0041 of JP2010-24113A canbe referred to. The content (filling percentage) of the non-magneticpowder of the non-magnetic layer is preferably 50 to 90 mass % and morepreferably 60 to 90 mass %.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the magnetic tape also includes asubstantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magneticsupport (hereinafter, also simply referred to as a “support”),well-known components such as polyethylene terephthalate, polyethylenenaphthalate, polyamide, polyamide imide, aromatic polyamide subjected tobiaxial stretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heatingtreatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic tape can also include a back coating layer includingnon-magnetic powder and a binding agent on a surface side of thenon-magnetic support opposite to the surface provided with the magneticlayer. The back coating layer preferably includes any one or both ofcarbon black and inorganic powder. In regards to the binding agentincluded in the back coating layer and various additives which can bearbitrarily included in the back coating layer, a well-known technologyregarding the treatment of the magnetic layer and/or the non-magneticlayer can be applied.

Various Thickness

A thickness of the non-magnetic support is preferably 3.00 to 6.00 μm.

A thickness of the magnetic layer is preferably equal to or smaller than0.15 μm and more preferably equal to or smaller than 0.10 μm, from aviewpoint of realization of high-density recording required in recentyears. The thickness of the magnetic layer is even more preferably 0.01to 0.10 μm. The magnetic layer may be at least single layer, themagnetic layer may be separated into two or more layers having differentmagnetic properties, and a configuration of a well-known multilayeredmagnetic layer can be applied. A thickness of the magnetic layer in acase where the magnetic layer is separated into two or more layers is atotal thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μmand is preferably 0.10 to 1.00 μm.

Meanwhile, the magnetic tape is normally used to be accommodated andcirculated in a magnetic tape cartridge. In order to increase recordingcapacity for 1 reel of the magnetic tape cartridge, it is desired toincrease a total length of the magnetic tape accommodated in 1 reel ofthe magnetic tape cartridge. In order to increase the recordingcapacity, it is necessary that the magnetic tape is thinned(hereinafter, referred to as “thinning”). As one method of thinning themagnetic tape, a method of decreasing a total thickness of a magneticlayer and a non-magnetic layer of a magnetic tape including thenon-magnetic layer and the magnetic layer on a non-magnetic support inthis order is used. In a case where the magnetic tape includes anon-magnetic layer, the total thickness of the magnetic layer and thenon-magnetic layer is preferably equal to or smaller than 1.80 μm, morepreferably equal to or smaller than 1.50 μm, and even more preferablyequal to or smaller than 1.10 μm, from a viewpoint of thinning themagnetic tape. In addition, the total thickness of the magnetic layerand the non-magnetic layer can be, for example, equal to or greater than0.10 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.90 μm and even more preferably 0.10 to 0.70 μm.

The thicknesses of various layers of the magnetic tape and thenon-magnetic support can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magnetic tapein a thickness direction is, for example, exposed by a well-known methodof ion beams or microtome, and the exposed cross section is observedwith a scanning electron microscope. In the cross section observation,various thicknesses can be acquired as a thickness acquired at oneposition of the cross section in the thickness direction, or anarithmetical mean of thicknesses acquired at a plurality of positions oftwo or more positions, for example, two positions which are arbitrarilyextracted. In addition, the thickness of each layer may be acquired as adesigned thickness calculated according to the manufacturing conditions.

Manufacturing Method

Preparation of Each Layer Forming Composition

Each composition for forming the magnetic layer, the non-magnetic layer,or the back coating layer normally includes a solvent, together withvarious components described above. As the solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. Among those, from a viewpoint ofsolubility of the binding agent normally used in the coating typemagnetic recording medium, each layer forming composition preferablyincludes one or more ketone solvents such as acetone, methyl ethylketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone,isophorone, and tetrahydrofuran. The amount of the solvent of each layerforming composition is not particularly limited, and can be set to bethe same as that of each layer forming composition of a typical coatingtype magnetic recording medium. In addition, steps of preparing eachlayer forming composition generally include at least a kneading step, adispersing step, and a mixing step provided before and after thesesteps, if necessary. Each step may be divided into two or more stages.All of raw materials used in the invention may be added at an initialstage or in a middle stage of each step. In addition, each raw materialmay be separately added in two or more steps. For example, a bindingagent may be separately added in a kneading step, a dispersing step, anda mixing step for adjusting viscosity after the dispersion. In amanufacturing step of the magnetic tape, a well-known manufacturingtechnology of the related art can be used in a part of the step or inthe entire step. In the kneading step, an open kneader, a continuouskneader, a pressure kneader, or a kneader having a strong kneading forcesuch as an extruder is preferably used. The details of the kneadingprocesses of these kneaders are disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-79274A (JP-H01-79274A). In addition, inorder to disperse each layer forming composition, glass beads and/orother beads can be used. As such dispersion beads, zirconia beads,titania beads, and steel beads which are dispersion beads having highspecific gravity are preferable. These dispersion beads are preferablyused by optimizing a bead diameter and a filling percentage. As adispersing machine, a well-known dispersing machine can be used. Eachlayer forming composition may be filtered by a well-known method beforeperforming the coating step. The filtering can be performed by using afilter, for example. As the filter used in the filtering, a filterhaving a hole diameter of 0.01 to 3 μm can be used, for example. Inaddition, as described above, as one method of obtaining a magnetic tapein which the ΔSFD calculated by Expression 1 is equal to or smaller than0.50, it is preferable that the dispersion conditions are reinforced (anincrease in dispersion time, a decrease in diameter and/or an increasein degree of filling of dispersion beads used in the dispersion, and thelike), and a dispersing agent is used.

Coating Step

The magnetic layer can be formed, for example, by directly applying themagnetic layer forming composition onto the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by applying the back coatinglayer forming composition to a side of the non-magnetic support oppositeto a side provided with the magnetic layer (or to be provided with themagnetic layer). For details of the coating for forming each layer, adescription disclosed in a paragraph 0066 of JP2010-231843A can bereferred to.

Other Steps

For details of various other steps for manufacturing the magnetic tape,descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A canbe referred to.

One Aspect of Preferred Manufacturing Method

As a preferred manufacturing method of the magnetic tape, amanufacturing method of applying vibration to the magnetic layer can beused, in order to improve uniformity of the thickness of the liquid filmof fatty acid ester on the surface of the magnetic layer. The inventorshave surmised that, by adding vibration, the liquid film of fatty acidester on the surface of the magnetic layer flows and the uniformity ofthe thickness of the liquid film is improved.

That is, the magnetic tape can be manufactured by a manufacturing methodof forming the magnetic layer by applying the magnetic layer formingcomposition including ferromagnetic powder, a binding agent, and fattyacid ester on the non-magnetic support and drying to form a magneticlayer, and applying vibration to the formed magnetic layer. Themanufacturing method is identical to the typical manufacturing method ofthe magnetic tape, except for applying vibration to the magnetic layer,and the details thereof are as described above.

Means for applying vibration are not particularly limited. For example,the vibration can be applied to the magnetic layer, by bringing thesurface of the non-magnetic support, provided with the magnetic layerformed, on a side opposite to the magnetic layer to come into contactwith a vibration imparting unit. The non-magnetic support, provided withthe magnetic layer formed, may run while coming into contact with avibration imparting unit. The vibration imparting unit, for example,includes an ultrasonic vibrator therein, and accordingly, vibration canbe applied to a product coming into contact with the unit. It ispossible to adjust the vibration applied to the magnetic layer by avibration frequency, and strength of the ultrasonic vibrator, and/or thecontact time with the vibration imparting unit. For example, the contacttime can be adjusted by a running speed of the non-magnetic support,provided with the magnetic layer formed, while coming into contact withthe vibration imparting unit. The vibration imparting conditions are notparticularly limited, and may be set so as to control the full width athalf maximum of the spacing distribution, particularly, the full widthat half maximum of the spacing distribution FWHM_(before) before vacuumheating. In order to set the vibration imparting conditions, apreliminary experiment can be performed before the actual manufacturing,and the conditions can be optimized.

In addition, the full width at half maximum of the spacing distributionFWHM_(after) after the vacuum heating tends to be decreased, in a casewhere the dispersion conditions of the magnetic layer formingcomposition are reinforced (for example, the number of times of thedispersion is increased, the dispersion time is extended, and the like),and/or the filtering conditions are reinforced (for example, a filterhaving a small hole diameter is used as a filter used in the filtering,the number of times of the filtering is increased, and the like). Theinventors have surmised that this is because the uniformity of theheight of the projection present on the surface of the magnetic layer isimproved, by improving dispersibility and/or the uniformity of theparticle size of the particulate matter included in the magnetic layerforming composition, particularly, the non-magnetic filler which mayfunction as the projection formation agent described above. Apreliminary experiment can be performed before the actual manufacturing,and the dispersion conditions and/or the filtering conditions can beoptimized.

In addition, in the magnetic tape including the magnetic layer includingcarbon black, it is effective to use the dispersing agent for improvingdispersibility of the carbon black as a magnetic layer component, inorder to decrease the full width at half maximum of the spacingdistribution FWHM_(after) after the vacuum heating. For example, organictertiary amine can be used as a dispersing agent of carbon black. Fordetails of the organic tertiary amine, descriptions disclosed inparagraphs 0011 to 0018 and 0021 of JP2013-049832A can be referred to.The organic tertiary amine is more preferably trialkylamine. An alkylgroup included in trialkylamine is preferably an alkyl group having 1 to18 carbon atoms. Three alkyl groups included in trialkylamine may be thesame as each other or different from each other. For details of thealkyl group, descriptions disclosed in paragraphs 0015 and 0016 ofJP2013-049832A can be referred to. As trialkylamine, trioctylamine isparticularly preferable.

As described above, it is possible to obtain a magnetic tape included inthe magnetic tape device according to one aspect of the invention.However, the manufacturing method described above is merely an example,the magnetic layer surface roughness Ra, the FWHM_(before), theFWHM_(after), the difference (S_(after)−S_(before)), and ΔSFD can becontrolled to be in respective ranges described above by an arbitrarymethod capable of adjusting the magnetic layer surface roughness Ra, theFWHM_(before), the FWHM_(after), the difference (S_(after)−S_(before)),and ΔSFD, and such an aspect is also included in the invention.

Formation of Servo Pattern

A servo pattern is formed in the magnetic layer by magnetizing aspecific position of the magnetic layer with a servo pattern recordinghead (also referred to as a “servo write head”). A well-known technologyregarding a servo pattern of the magnetic layer of the magnetic tapewhich is well known can be applied for the shapes of the servo patternwith which the head tracking servo can be performed and the dispositionthereof in the magnetic layer. For example, as a head tracking servosystem, a timing-based servo system and an amplitude-based servo systemare known. The servo pattern of the magnetic layer of the magnetic tapemay be a servo pattern capable of allowing head tracking servo of anysystem. In addition, a servo pattern capable of allowing head trackingservo in the timing-based servo system and a servo pattern capable ofallowing head tracking servo in the amplitude-based servo system may beformed in the magnetic layer.

The magnetic tape described above is generally accommodated in amagnetic tape cartridge and the magnetic tape cartridge is mounted inthe magnetic tape device. In the magnetic tape cartridge, the magnetictape is generally accommodated in a cartridge main body in a state ofbeing wound around a reel. The reel is rotatably provided in thecartridge main body. As the magnetic tape cartridge, a single reel typemagnetic tape cartridge including one reel in a cartridge main body anda twin reel type magnetic tape cartridge including two reels in acartridge main body are widely used. In a case where the single reeltype magnetic tape cartridge is mounted in the magnetic tape device(drive) in order to record and/or reproduce information (magneticsignals) to the magnetic tape, the magnetic tape is drawn from themagnetic tape cartridge and wound around the reel on the drive side. Aservo head is disposed on a magnetic tape transportation path from themagnetic tape cartridge to a winding reel. Sending and winding of themagnetic tape are performed between a reel (supply reel) on the magnetictape cartridge side and a reel (winding reel) on the drive side. In themeantime, the servo head comes into contact with and slides on thesurface of the magnetic layer of the magnetic tape, and accordingly, thereading of the servo pattern is performed by the servo head. Withrespect to this, in the twin reel type magnetic tape cartridge, bothreels of the supply reel and the winding reel are provided in themagnetic tape cartridge. The magnetic tape according to one aspect ofthe invention may be accommodated in any of single reel type magnetictape cartridge and twin reel type magnetic tape cartridge. Theconfiguration of the magnetic tape cartridge is well known.

Servo Head

The magnetic tape device includes the TMR head as the servo head. TheTMR head is a magnetic head including a tunnel magnetoresistance effecttype element (TMR element). The TMR element can play a role of detectinga change in leakage magnetic field from the magnetic tape as a change inresistance value (electric resistance) by using a tunnelmagnetoresistance effect, as a servo pattern reading element for readinga servo pattern formed in the magnetic layer of the magnetic tape. Byconverting the detected change in resistance value into a change involtage, the servo pattern can be read (servo signal can be reproduced).

As the TMR head included in the magnetic tape device, a TMR head havinga well-known configuration including a tunnel magnetoresistance effecttype element (TMR element) can be used. For example, for details of thestructure of the TMR head, materials of each unit configuring the TMRhead, and the like, well-known technologies regarding the TMR head canbe used.

The TMR head is a so-called thin film head. The TMR element included inthe TMR head at least includes two electrode layers, a tunnel barrierlayer, a free layer, and a fixed layer. The TMR head includes a TMRelement in a state where cross sections of these layers face a side of asurface sliding on the magnetic tape. The tunnel barrier layer ispositioned between the two electrode layers and the tunnel barrier layeris an insulating layer. Meanwhile, the free layer and the fixed layerare magnetic layers. The free layer is also referred to as amagnetization free layer and is a layer in which a magnetizationdirection changes depending on the external magnetic field. On the otherhand, the fixed layer is a layer in which a magnetization direction doesnot change depending on the external magnetic field. The tunnel barrierlayer (insulating layer) is positioned between the two electrodes,normally, and thus, even in a case where a voltage is applied, ingeneral, a current does not flow or does not substantially flow.However, a current (tunnel current) flows by the tunnel effect dependingon a magnetization direction of the free layer affected by a leakagemagnetic field from the magnetic tape. The amount of a tunnel currentflow changes depending on a relative angle of a magnetization directionof the fixed layer and a magnetization direction of the free layer, andas the relative angle decreases, the amount of the tunnel current flowincreases. A change in amount of the tunnel current flow is detected asa change in resistance value by the tunnel magnetoresistance effect. Byconverting the change in resistance value into a change in voltage, theservo pattern can be read. For an example of the configuration of theTMR head, a description disclosed in FIG. 1 of JP2004-185676A can bereferred to, for example. However, there is no limitation to the aspectshown in the drawing. FIG. 1 of JP2004-185676A shows two electrodelayers and two shield layers. Here, a TMR head having a configuration inwhich the shield layer serves as an electrode layer is also well knownand the TMR head having such a configuration can also be used. In theTMR head, a current (tunnel current) flows between the two electrodesand thereby changing electric resistance, by the tunnelmagnetoresistance effect. The TMR head is a magnetic head having a CPPstructure, and thus, a direction in which a current flows is atransportation direction of the magnetic tape. In the invention and thespecification, the description regarding “orthogonal” includes a rangeof errors allowed in the technical field of the invention. For example,the range of errors means a range of less than ±10° from an exactorthogonal state, and the error from the exact orthogonal state ispreferably within ±5° and more preferably within ±3°. A decrease inresistance value of the TMR head means a decrease in electric resistancemeasured by bringing an electric resistance measuring device intocontact with a wiring connecting two electrodes, and a decrease inelectric resistance between two electrodes in a state where a currentdoes not flow. A significant decrease in resistance value (electricresistance) tends to become significant at the time of reading a servopattern written in the magnetic layer of magnetic tape including themagnetic layer having the magnetic layer surface roughness Ra equal toor smaller than 2.0 nm. However, such a significant decrease inresistance value can be prevented by setting the FWHM_(before), theFWHM_(after), and the difference (S_(after)−S_(before)) to be in therange described above, in the magnetic tape in which the magnetic layersurface roughness Ra is equal to or smaller than 2.0 nm.

In one preferred aspect, in the magnetic tape device, it is possible toperform the head tracking servo by using the TMR head as the servo headin a case of recording information on the magnetic layer having a servopattern at linear recording density equal to or greater than 250 kfciand/or reproducing information recorded. The unit, kfci, is a unit oflinear recording density (not able to convert to the SI unit system).The linear recording density can be, for example, equal to or greaterthan 250 kfci and can also be equal to or greater than 300 kfci. Thelinear recording density can be, for example, equal to or smaller than800 kfci and can also exceed 800 kfci. In the magnetic tape forhigh-density recording, a width of the servo band tends to decrease, inorder to provide a large amount of data bands in the magnetic layer, andthus, the SNR at the time of reading a servo pattern easily decrease.However, a decrease in SNR can be prevented by setting the magneticlayer surface roughness Ra and the ΔSFD of the magnetic tape in themagnetic tape device to be in the ranges described above.

The servo head is a magnetic head including at least the TMR element asa servo pattern reading element. The servo head may include or may notinclude a reproducing element for reproducing information recorded onthe magnetic tape. That is, the servo head and the reproducing head maybe one magnetic head or separated magnetic heads. The same applies to arecording element for performing the recording of information in themagnetic tape.

As the magnetic tape is transported at a high speed in the magnetic tapedevice, it is possible to shorten the time for recording informationand/or the time for reproducing information. Meanwhile, it is desiredthat the magnetic tape is transported at a low speed at the time ofrecording and reproducing information, in order to prevent adeterioration in recording and reproducing characteristics. From theviewpoint described above, in a case of reading a servo pattern by theservo head in order to perform head tracking servo at the time ofrecording and/or reproducing information, a magnetic tape transportationspeed is preferably equal to or lower than 18 m/sec, more preferablyequal to or lower than 15 m/sec, and even more preferably equal to orlower than 10 m/sec. The magnetic tape transportation speed can be, forexample, equal to or higher than 1 m/sec. The magnetic tapetransportation speed is also referred to as a running speed. In theinvention and the specification, the “magnetic tape transportationspeed” is a relative speed between the magnetic tape transported in themagnetic tape device and the servo head in a case where the servopattern is read by the servo head. The magnetic tape transportationspeed is normally set in a control unit of the magnetic tape device. Asthe magnetic tape transportation speed is low, the time for which thesame portion of the TMR head comes into contact with the magnetic tapeincreases at the time of reading the servo pattern, and accordingly,damage on the TMR head more easily occurs and a decrease in resistancevalue easily occurs. In the magnetic tape device according to one aspectof the invention, such a decrease in resistance value can be preventedby using the magnetic tape described above.

Head Tracking Servo Method

One aspect of the invention relates to a head tracking servo methodincluding: reading a servo pattern of a magnetic layer of a magnetictape by a servo head in a magnetic tape device, in which the servo headis a magnetic head including a tunnel magnetoresistance effect typeelement as a servo pattern reading element, the magnetic tape includes anon-magnetic support, and a magnetic layer including ferromagneticpowder, a binding agent, and fatty acid ester on the non-magneticsupport, the magnetic layer includes a servo pattern, a center lineaverage surface roughness Ra measured regarding a surface of themagnetic layer is equal to or smaller than 2.0 nm, a full width at halfmaximum of spacing distribution measured by optical interferometryregarding the surface of the magnetic layer before performing a vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 7.0 nm, a full width at half maximum of spacingdistribution measured by optical interferometry regarding the surface ofthe magnetic layer after performing the vacuum heating with respect tothe magnetic tape is greater than 0 nm and equal to or smaller than 7.0nm, a difference (S_(after)−S_(before)) between a spacing S_(after)measured by optical interferometry regarding the surface of the magneticlayer after performing the vacuum heating with respect to the magnetictape and a spacing S_(before) measured by optical interferometryregarding the surface of the magnetic layer before performing the vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 8.0 nm, and ΔSFD in a longitudinal direction of themagnetic tape calculated by Expression 1 is equal to or smaller than0.50. The reading of the servo pattern is performed by bringing themagnetic tape into contact with the servo head allowing sliding whiletransporting (causing running of) the magnetic tape. The details of themagnetic tape and the servo head used in the head tracking servo methodare as the descriptions regarding the magnetic tape device according toone aspect of the invention.

Hereinafter, as one specific aspect of the head tracking servo, headtracking servo in the timing-based servo system will be described.However, the head tracking servo of the invention is not limited to thefollowing specific aspect.

In the head tracking servo in the timing-based servo system(hereinafter, referred to as a “timing-based servo”), a plurality ofservo patterns having two or more different shapes are formed in amagnetic layer, and a position of a servo head is recognized by aninterval of time in a case where the servo head has read the two servopatterns having different shapes and an interval of time in a case wherethe servo head has read two servo patterns having the same shapes. Theposition of the magnetic head of the magnetic tape in the widthdirection is controlled based on the position of the servo headrecognized as described above. In one aspect, the magnetic head, theposition of which is controlled here, is a magnetic head (reproducinghead) which reproduces information recorded on the magnetic tape, and inanother aspect, the magnetic head is a magnetic head (recording head)which records information in the magnetic tape.

FIG. 1 shows an example of disposition of data bands and servo bands. InFIG. 1, a plurality of servo bands 10 are disposed to be interposedbetween guide bands 12 in a magnetic layer of a magnetic tape 1. Aplurality of regions 11 each of which is interposed between two servobands are data bands. The servo pattern is a magnetized region and isformed by magnetizing a specific region of the magnetic layer by a servowrite head. The region magnetized by the servo write head (positionwhere a servo pattern is formed) is determined by standards. Forexample, in an LTO Ultrium format tape which is based on a localstandard, a plurality of servo patterns tilted in a tape width directionas shown in FIG. 2 are formed on a servo band in a case of manufacturinga magnetic tape. Specifically, in FIG. 2, a servo frame SF on the servoband 10 is configured with a servo sub-frame 1 (SSF1) and a servosub-frame 2 (SSF2). The servo sub-frame 1 is configured with an A burst(in FIG. 2, reference numeral A) and a B burst (in FIG. 2, referencenumeral B). The A burst is configured with servo patterns A1 to A5 andthe B burst is configured with servo patterns B1 to B5. Meanwhile, theservo sub-frame 2 is configured with a C burst (in FIG. 2, referencenumeral C) and a D burst (in FIG. 2, reference numeral D). The C burstis configured with servo patterns C1 to C4 and the D burst is configuredwith servo patterns D1 to D4. Such 18 servo patterns are disposed in thesub-frames in the arrangement of 5, 5, 4, 4, as the sets of 5 servopatterns and 4 servo patterns, and are used for recognizing the servoframes. FIG. 2 shows one servo frame for explaining. However, inpractice, in the magnetic layer of the magnetic tape in which the headtracking servo in the timing-based servo system is performed, aplurality of servo frames are disposed in each servo band in a runningdirection. In FIG. 2, an arrow shows the running direction. For example,an LTO Ultrium format tape generally includes 5,000 or more servo framesper a tape length of 1 m, in each servo band of the magnetic layer. Theservo head sequentially reads the servo patterns in the plurality ofservo frames, while coming into contact with and sliding on the surfaceof the magnetic layer of the magnetic tape transported in the magnetictape device.

In the head tracking servo in the timing-based servo system, a positionof a servo head is recognized based on an interval of time in a casewhere the servo head has read the two servo patterns (reproduced servosignals) having different shapes and an interval of time in a case wherethe servo head has read two servo patterns having the same shapes. Thetime interval is normally obtained as a time interval of a peak of areproduced waveform of a servo signal. For example, in the aspect shownin FIG. 2, the servo pattern of the A burst and the servo pattern of theC burst are servo patterns having the same shapes, and the servo patternof the B burst and the servo pattern of the D burst are servo patternshaving the same shapes. The servo pattern of the A burst and the servopattern of the C burst are servo patterns having the shapes differentfrom the shapes of the servo pattern of the B burst and the servopattern of the D burst. An interval of the time in a case where the twoservo patterns having different shapes are read by the servo head is,for example, an interval between the time in a case where any servopattern of the A burst is read and the time in a case where any servopattern of the B burst is read. An interval of the time in a case wherethe two servo patterns having the same shapes are read by the servo headis, for example, an interval between the time in a case where any servopattern of the A burst is read and the time in a case where any servopattern of the C burst is read. The head tracking servo in thetiming-based servo system is a system supposing that occurrence of adeviation of the time interval is due to a position change of themagnetic tape in the width direction, in a case where the time intervalis deviated from the set value. The set value is a time interval in acase where the magnetic tape runs without occurring the position changein the width direction. In the timing-based servo system, the magnetichead is moved in the width direction in accordance with a degree of thedeviation of the obtained time interval from the set value.Specifically, as the time interval is greatly deviated from the setvalue, the magnetic head is greatly moved in the width direction. Thispoint is applied to not only the aspect shown in FIGS. 1 and 2, but alsoto entire timing-based servo systems.

For the details of the head tracking servo in the timing-based servosystem, well-known technologies such as technologies disclosed in U.S.Pat. No. 5,689,384A, U.S. Pat. No. 6,542,325B, and U.S. Pat. No.7,876,521B can be referred to, for example. In addition, for the detailsof the head tracking servo in the amplitude-based servo system,well-known technologies disclosed in U.S. Pat. No. 5,426,543A and U.S.Pat. No. 5,898,533A can be referred to, for example.

According to one aspect of the invention, a magnetic tape used in amagnetic tape device in which a TMR head is used as a servo head, themagnetic tape including: a magnetic layer including ferromagneticpowder, a binding agent, and fatty acid ester on a non-magnetic support,in which the magnetic layer includes a servo pattern, a center lineaverage surface roughness Ra measured regarding a surface of themagnetic layer is equal to or smaller than 2.0 nm, a full width at halfmaximum of spacing distribution measured by optical interferometryregarding the surface of the magnetic layer before performing a vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 7.0 nm, a full width at half maximum of spacingdistribution measured by optical interferometry regarding the surface ofthe magnetic layer after performing the vacuum heating with respect tothe magnetic tape is greater than 0 nm and equal to or smaller than 7.0nm, and a difference (S_(after)−S_(before)) between a spacing S_(after)measured by optical interferometry regarding the surface of the magneticlayer after performing the vacuum heating with respect to the magnetictape and a spacing S_(before) measured by optical interferometryregarding the surface of the magnetic layer before performing the vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 8.0 nm, and ΔSFD in a longitudinal direction of themagnetic tape calculated by Expression 1 is equal to or smaller than0.50, is also provided. The details of the magnetic tape are also as thedescriptions regarding the magnetic tape device according to one aspectof the invention.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and“mass %”, unless otherwise noted. In addition, steps and evaluationsdescribed below are performed in an environment of an atmospheretemperature of 23° C.±1° C., unless otherwise noted.

Example 1

1. Manufacturing of Magnetic Tape

(1) Preparation of Alumina Dispersion

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixedsolvent of methyl ethyl ketone and toluene) of a polyester polyurethaneresin having a SO₃Na group as a polar group (UR-4800 (amount of a polargroup: 80 meq/kg) manufactured by Toyobo Co., Ltd.), and 570.0 parts ofa mixed solution of methyl ethyl ketone and cyclohexanone (mass ratio of1:1) as a solvent were mixed in 100.0 parts of alumina powder (HIT-80manufactured by Sumitomo Chemical Co., Ltd.) having an gelatinizationratio of 65% and a BET specific surface area of 30 m²/g, and dispersedin the presence of zirconia beads by a paint shaker for 5 hours. Afterthe dispersion, the dispersion liquid and the beads were separated by amesh and an alumina dispersion was obtained.

(2) Magnetic Layer Forming Composition List

Magnetic Solution

Ferromagnetic powder (Ferromagnetic hexagonal ferrite powder (bariumferrite)): 100.0 parts

Average particle size, coercivity Hc, and ΔSFD_(powder) calculated byExpression 2: see Table 1

SO₃Na group-containing polyurethane resin: 14.0 parts

Weight-average molecular weight: 70,000, SO₃Na group: 0.2 meq/g

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

Abrasive Liquid

Alumina dispersion prepared in the section (1): 6.0 parts

Silica Sol (Projection forming agent liquid)

Colloidal silica: 2.0 parts

Average particle size: see Table 1

Methyl ethyl ketone: 1.4 parts

Other Components

Butyl stearate: see Table 1

Stearic acid: 1.0 part

Polyisocyanate (CORONATE (registered trademark) manufactured by NipponPolyurethane Industry Co., Ltd.): 2.5 parts

Finishing Additive Solvent

Cyclohexanone: 200.0 parts

Methyl ethyl ketone: 200.0 parts

(3) Non-Magnetic Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

Average particle size (average long axis length): 0.15 μm

Average acicular ratio: 7

BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

Average particle size: 20 nm

An electron beam-curable vinyl chloride copolymer: 13.0 parts

An electron beam-curable polyurethane resin: 6.0 parts

Butyl stearate: see Table 1

Stearic acid: 1.0 part

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

(4) List of Components of Back Coating Layer Forming Composition

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

Average particle size (average long axis length): 0.15 μm

Average acicular ratio: 7

BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

A sulfonic acid group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate (CORONATE (registered trademark) L manufactured by NipponPolyurethane Industry Co., Ltd.): 5.0 parts

Methyl ethyl ketone: 155.0 parts

Cyclohexanone: 355.0 parts

(5) Preparation of Each Layer Forming Composition

(i) Preparation of Magnetic Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

A magnetic solution was prepared by performing beads-dispersing of themagnetic solution components described above by using beads as thedispersion medium in a batch type vertical sand mill. The dispersiontime of the beads dispersion was set as the dispersion time shown inTable 1 and zirconia beads having a bead diameter of 0.5 mm were used asthe dispersion beads.

The prepared magnetic solution, the abrasive liquid and other components(the silica sol, the other components, and the finishing additivesolvents) were introduced into a dissolver stirrer, and were stirred ata circumferential speed of 10 m/sec for 30 minutes. After that, thetreatment was performed with a flow type ultrasonic dispersing device ata flow rate of 7.5 kg/min for the number of times of the passes shown inTable 1, and then, a magnetic layer forming composition was prepared byperforming filtering with a filter having a hole diameter shown in Table1, for the number of times of the passes shown in Table 1. A part of theprepared magnetic layer forming composition was collected and adispersion particle diameter which is an index for dispersibility offerromagnetic powder (ferromagnetic hexagonal barium ferrite powder) wasmeasured by a method which will be described later. The measured valueis shown in Table 1.

(ii) Preparation of Non-Magnetic Layer Forming Composition

The non-magnetic layer forming composition was prepared by the followingmethod.

Each component excluding stearic acid, butyl stearate, cyclohexanone,and methyl ethyl ketone was beads-dispersed by using batch type verticalsand mill for 24 hours to obtain a dispersion liquid. As the dispersionbeads, zirconia beads having a bead diameter of 0.1 mm were used. Afterthat, the remaining components were added into the obtained dispersionliquid and stirred with a dissolver. The dispersion liquid obtained asdescribed above was filtered with a filter having a hole diameter of 0.5μm and a non-magnetic layer forming composition was prepared.

(iii) Preparation of Back Coating Layer Forming Composition

The back coating layer forming composition was prepared by the followingmethod.

Each component excluding stearic acid, butyl stearate, polyisocyanate,and cyclohexanone was kneaded and diluted by an open kneader. Then, theobtained mixed solution was subjected to a dispersion process of 12passes, with a transverse beads mill dispersing device and zirconiabeads having a bead diameter of 1 mm, by setting a bead fillingpercentage as 80 volume %, a circumferential speed of rotor distal endas 10 m/sec, and a retention time for 1 pass as 2 minutes. After that,the remaining components were added into the obtained dispersion liquidand stirred with a dissolver. The dispersion liquid obtained asdescribed above was filtered with a filter having a hole diameter of 1.0μm and a back coating layer forming composition was prepared.

(6) Manufacturing Method of Magnetic Tape

The non-magnetic layer forming composition was applied onto apolyethylene naphthalate support having a thickness of 5.00 μm and driedso that the thickness after the drying becomes 1.00 μm, and then, anelectron beam was emitted with an energy of 40 kGy at an accelerationvoltage of 125 kV. The magnetic layer forming composition was applied sothat the thickness after the drying becomes 70 nm (0.07 μm) to form acoating layer of the magnetic layer forming composition. The formedcoating layer was dried without performing the orientation process(non-orientation).

After that, the support, provided with the coating layer formed, wasinstalled in a vibration imparting device shown in FIG. 3 so that thesurface thereof on a side opposite to the surface where the coatinglayer is formed comes into contact with the vibration imparting unit,and the support (in FIG. 3, reference numeral 101), provided with thecoating layer formed, was transported at a transportation speed of 0.5m/sec, to apply vibration to the coating layer. In FIG. 3, a referencenumeral 102 denotes a guide roller (a reference numeral 102 denotes oneof two guide rollers), a reference numeral 103 denotes the vibrationimparting device (vibration imparting unit including the ultrasonicvibrator), and an arrow denotes a transportation direction. The timefrom the start of the contact of the arbitrary portion of the support,provided with the coating layer formed, with the vibration impartingunit until the end of the contact (vibration imparting time) is shown inTable 1 as the imparting time. The vibration imparting unit usedincludes an ultrasonic vibrator therein. The vibration was imparted bysetting a vibration frequency and the intensity of the ultrasonicvibrator as values shown in Table 1.

After that, the back coating layer forming composition was applied ontothe surface of the support on a side opposite to the surface where thenon-magnetic layer and the magnetic layer are formed, and dried so thatthe thickness after the drying becomes thickness of 0.40 μm.

After that, the surface smoothing treatment (calender process) wasperformed with a a calender roll configured of only a metal roll, at acalender process speed of 80 m/min, linear pressure of 300 kg/cm (294kN/m), and a calender temperature (surface temperature of a calenderroll) shown in Table 1. As the calender process conditions arereinforced (for example, as the surface temperature of the calender rollincreases), the center line average surface roughness Ra measuredregarding the surface of the magnetic layer tends to decrease. Then, thethermal treatment was performed in the environment of the atmospheretemperature of 70° C. for 36 hours. After the thermal treatment, theslitting was performed so as to have a width of ½ inches (0.0127meters), and the surface of the magnetic layer was cleaned with a tapecleaning device in which a nonwoven fabric and a razor blade areattached to a device including a sending and winding device of the slitso as to press the surface of the magnetic layer.

By doing so, a magnetic tape for forming a servo pattern in the magneticlayer was manufactured.

In a state where the magnetic layer of the manufactured magnetic tapewas demagnetized, servo patterns having disposition and shapes accordingto the LTO Ultrium format were formed on the magnetic layer by using aservo write head mounted on a servo tester. Accordingly, a magnetic tapeincluding data bands, servo bands, and guide bands in the dispositionaccording to the LTO Ultrium format in the magnetic layer, and includingservo patterns having the disposition and the shape according to the LTOUltrium format on the servo band is manufactured. The servo testerincludes a servo write head and a servo head. This servo tester was alsoused in evaluations which will be described later.

The thickness of each layer and the thickness of the non-magneticsupport of the manufactured magnetic tape were acquired by the followingmethod, and it was confirmed that the thicknesses obtained are thethicknesses described above.

A cross section of the magnetic tape in a thickness direction wasexposed to ion beams and the exposed cross section was observed with ascanning electron microscope. Various thicknesses were obtained as anarithmetical mean of thicknesses obtained at two portions in thethickness direction in the cross section observation.

A part of the magnetic tape manufactured by the method described abovewas used in the evaluation of physical properties described below, andthe other part was used in order to measure an SNR and a resistancevalue of the TMR head which will be described later.

2. Evaluation of Ferromagnetic Powder and Magnetic Layer FormingComposition

(1) Dispersion Particle Diameter of Magnetic Layer Forming Composition

A part of the magnetic layer forming composition prepared as describedabove was collected, and a sample solution diluted by an organic solventused in the preparation of the composition to 1/50 based on mass wasprepared. Regarding the prepared sample solution, an arithmetic averageparticle diameter measured by using a light scattering particle sizeanalyzer (LB500 manufactured by Horiba, Ltd.) was used as the dispersionparticle diameter.

(2) Average Particle Size of Ferromagnetic Powder

An average particle size of the ferromagnetic powder was obtained by themethod described above.

(3) ΔSFD_(powder) and Coercivity Hc of Ferromagnetic Powder

Regarding the ferromagnetic powder, the SFDs were measured at atemperature of 100° C. and a temperature of 25° C. with an appliedmagnetic field of 796 kA/m (10 kOe) by using an oscillation sample typemagnetic-flux meter (manufactured by Toei Industry Co., Ltd.). Frommeasurement results of the SFDs, the ΔSFD_(powder) was calculated byExpression 2.

The coercivity Hc of the ferromagnetic powder was measured at atemperature of 25° C. with an applied magnetic field of 796 kA/m (10kOe) by using an oscillation sample type magnetic-flux meter(manufactured by Toei Industry Co., Ltd.).

The evaluation was performed in Examples and Comparative Examples whichwill be described later in the same manner as described above.

3. Evaluation of Physical Properties of Magnetic Tape

(1) Center Line Average Surface Roughness Ra Measured Regarding Surfaceof Magnetic Layer

The measurement regarding a measurement area of 40 μm×40 μm in thesurface of the magnetic layer of the magnetic tape was performed with anatomic force microscope (AFM, Nanoscope 4 manufactured by VeecoInstruments, Inc.) in a tapping mode, and a center line average surfaceroughness Ra was acquired. RTESP-300 manufactured by BRUKER is used as aprobe, a scan speed (probe movement speed) was set as 40 μm/sec, and aresolution was set as 512 pixel×512 pixel.

(2) Full Width at Half Maximum of Spacing Distributions FWHM_(before)and FWHM_(after) Before and After Vacuum Heating

The full width at half maximum of the spacing distributionsFWHM_(before) and FWHM_(after) before and after vacuum heating wereacquired by the following method by using a tape spacing analyzer (TSA)(manufactured by Micro Physics, Inc.).

In a state where a glass sheet included in the TSA was disposed on thesurface of the magnetic layer of the magnetic tape, a hemisphere waspressed against the surface of the back coating layer of the magnetictape at a pressure of 5.05×10⁴ N/m (0.5 atm) by using a hemisphere madeof urethane included in the TSA as a pressing member. In this state, agiven region (150,000 to 200,000 μm²) of the surface of the magneticlayer of the magnetic tape was irradiated with white light from astroboscope included in the TSA through the glass sheet, and theobtained reflected light was received by a charge-coupled device (CCD)through an interference filter (filter selectively passing light at awavelength of 633 nm), and thus, an interference fringe image generatedon the uneven part of the region was obtained.

This image was divided into 300,000 points, a distance (spacing) betweenthe surface of the glass sheet on the magnetic tape side and the surfaceof the magnetic layer of the magnetic tape was acquired, and the fullwidth at half maximum of spacing distribution was full width at halfmaximum, in a case where this spacing was shown with a histogram, andthis histogram was fit with Gaussian distribution.

The vacuum heating was performed by storing the magnetic tape in avacuum constant temperature drying machine with a degree of vacuum of200 Pa to 0.01 Mpa and at inner atmosphere temperature of 70° C. to 90°C. for 24 hours.

(3) Difference (S_(after)−S_(before))

The difference (S_(after)−S_(before)) was a value obtained bysubtracting a mode of the histogram before the vacuum heating from amode of the histogram after the vacuum heating obtained in the section(2).

(4) ΔSFD

The SFDs were measured in a longitudinal direction of the magnetic tapeat a temperature of 25° C. and a temperature of −190° C. with an appliedmagnetic field of 796 kA/m (10 kOe) by using an oscillation sample typemagnetic-flux meter (manufactured by Toei Industry Co., Ltd.). Frommeasurement results, the ΔSFD in a longitudinal direction of themagnetic tape was calculated by Expression 1.

4. Measurement of SNR

In an environment of an atmosphere temperature of 23° C.±1° C. andrelative humidity of 50%, the servo head of the servo tester wasreplaced with a commercially available TMR head (element width of 70 nm)as a reproducing head for HDD. The reading of a servo pattern wasperformed by attaching the magnetic tape manufactured in the section 1.to the servo tester, and the SNR was obtained as a ratio of the outputand noise. The SNR was calculated as a relative value by setting the SNRmeasured as 0 dB in Comparative Example 1 which will be described later.In a case where the SNR calculated as described above is equal to orgreater than 7.0 dB, it is possible to evaluate that performance ofdealing with future needs accompanied with high-density recording isobtained.

5. Measurement of Resistance Value of Servo Head

In an environment of an atmosphere temperature of 23° C.±1° C. andrelative humidity of 50%, the servo head of the servo tester wasreplaced with a commercially available TMR head (element width of 70 nm)as a reproducing head for HDD. In the servo tester, the magnetic tapemanufactured in the part 1. was transported while bringing the surfaceof the magnetic layer into contact with the servo head and causingsliding therebetween. A tape length of the magnetic tape was 1,000 m,and a total of 4,000 passes of the transportation (running) of themagnetic tape was performed by setting the magnetic tape transportationspeed (relative speed of the magnetic tape and the servo head) at thetime of the transportation as 4 m/sec. The servo head was moved in awidth direction of the magnetic tape by 2.5 μm for 1 pass, a resistancevalue (electric resistance) of the servo head for transportation of 400passes was measured, and a rate of a decrease in resistance value withrespect to an initial value (resistance value at 0 pass) was obtained bythe following equation.

Rate of decrease in resistance value(%)=[(initial value−resistance valueafter transportation of 400 passes)/initial value]×100

The measurement of the resistance value (electric resistance) wasperformed by bringing an electric resistance measuring device (digitalmulti-meter (product number: DA-50C) manufactured by Sanwa ElectricInstrument Co., Ltd.) into contact with a wiring connecting twoelectrodes of a TMR element included in a TMR head. In a case where thecalculated rate of a decrease in resistance value was equal to orgreater than 30%, it was determined that a decrease in resistance valueoccurred. Then, a servo head was replaced with a new head, andtransportation after 400 passes was performed and a resistance value wasmeasured. The number of times of occurrence of a decrease in resistancevalue which is 1 or greater indicates a significant decrease inresistance value. In the running of 4,000 passes, in a case where therate of a decrease in resistance value did not become equal to orgreater than 30%, the number of times of occurrence of a decrease inresistance value was set as 0. In a case where the number of times ofoccurrence of a decrease in resistance value is 0, the maximum value ofthe measured rate of a decrease in resistance value is shown in Table 1.

Examples 2 to 8 and Comparative Examples 1 to 13

1. Manufacturing of Magnetic Tape

A magnetic tape was manufactured in the same manner as in Example 1,except that various conditions shown in Table 1 were changed as shown inTable 1.

In Table 1, in the comparative examples in which “none” is shown in acolumn of the orientation, the magnetic layer was formed withoutperforming the orientation process in the same manner as in Example 1.

In the examples in which “longitudinal” is disclosed in a column of theorientation, a longitudinal orientation process was performed byapplying a magnetic field having a magnetic field strength of 0.3 T tothe surface of the coating layer in a longitudinal direction, while thecoating layer of the magnetic layer forming composition is wet. Afterthat, the coating layer was dried.

In Table 1, in the comparative examples in which “none” is disclosed ina column of the ultrasonic vibration imparting conditions, a magnetictape was manufactured by a manufacturing step in which the vibrationimparting is not performed.

2. Evaluation of Physical Properties of Magnetic Tape

Various physical properties of the manufactured magnetic tape wereevaluated in the same manner as in Example 1.

3. Measurement of SNR

The SNR was measured by the same method as that in Example 1, by usingthe manufactured magnetic tape. In Examples 2 to 8 and ComparativeExamples 5 to 13, the TMR head which was the same as that in Example 1was used as a servo head. In Comparative Examples 1 to 4, a commerciallyavailable spin valve type GMR head (element width of 70 nm) was used asa servo head.

4. Measurement of Resistance Value of Servo Head

A resistance value of the servo head was measured by the same method asthat in Example 1, by using the manufactured magnetic tape. As the servohead, the same servo head (TMR head or GMR head) as the servo head usedin the measurement of the SNR was used. In Comparative Example 10, itwas difficult to continue the sliding between the magnetic tape and theservo head due to the sticking of the magnetic tape and the servo head.In Comparative Example 11, it was difficult to allow the sliding betweenthe magnetic tape and the servo head due to an effect of scrapsgenerated due to chipping of the surface of the magnetic layer caused bythe sliding between the magnetic tape and the servo head. Thus, inComparative Examples 10 and 11, the measurement of a resistance value ofthe servo head was not performed.

In Comparative Examples 1 to 4, the GMR head used as the servo head wasa magnetic head having a CIP structure including two electrodes with anMR element interposed therebetween in a direction orthogonal to thetransportation direction of the magnetic tape. A resistance value wasmeasured in the same manner as in Example 1, by bringing an electricresistance measuring device into contact with a wiring connecting thesetwo electrodes.

The results of the evaluations described above are shown in Table 1.

TABLE 1 Ferromagnetic hexagonal ferrite powder Dispersion Content ofAverage Beads Dispersion Colloidal silica butyl stearate particledispersion particle average particle Magnetic Hc size time diameter sizeCalender layer forming ΔSFD_(power) (Oe) (kA/m) (nm) (hour) (nm)Orientation (nm) temperature composition Comparative 0.30 1978 157 25 4820 None 120 nm  80° C. 1.0 part Example 1 Comparative 0.30 1978 157 2548 20 None 120 nm  90° C. 1.0 part Example 2 Comparative 0.30 1978 15725 48 20 None 80 nm 90° C. 1.0 part Example 3 Comparative 0.30 1978 15725 48 20 None 40 nm 110° C.  1.0 part Example 4 Comparative 0.30 1978157 25 48 20 None 120 nm  80° C. 1.0 part Example 5 Comparative 0.301978 157 25 48 20 None 120 nm  90° C. 1.0 part Example 6 Comparative0.30 1978 157 25 48 20 None 80 nm 90° C. 1.0 part Example 7 Comparative0.30 1978 157 25 48 20 None 40 nm 110° C.  1.0 part Example 8Comparative 0.20 2011 160 25 48 20 None 80 nm 90° C. 1.0 part Example 9Comparative 0.20 2011 160 25 48 20 None 80 nm 90° C. 1.0 part Example 10Comparative 0.20 2011 160 25 48 20 None 80 nm 90° C.   0 part Example 11Comparative 0.20 2011 160 25 48 20 None 80 nm 90° C. 1.0 part Example 12Comparative 0.30 1978 157 25 48 20 None 80 nm 90° C. 1.0 part Example 13Example 1 0.20 2011 160 25 48 20 None 80 nm 90° C. 1.0 part Example 20.80 1850 147 24 48 20 Longitudinal 80 nm 90° C. 1.0 part Example 3 0.301978 157 25 48 20 Longitudinal 80 nm 90° C. 1.0 part Example 4 0.10 1840146 23 35 50 Longitudinal 80 nm 90° C. 1.0 part Example 5 0.10 1840 14623 48 20 Longitudinal 80 mn 90° C. 1.0 part Example 6 0.30 1978 157 2548 20 Longitudinal 80 nm 90° C. 1.0 part Example 7 0.30 1978 157 25 4820 Longitudinal 80 nm 90° C. 1.0 part Example 8 0.30 1978 157 25 48 20Longitudinal 40 nm 110° C.  1.0 part Magnetic layer forming Content ofcomposition preparation conditions butyl stearate Number of timesNon-magnetic Ultrasonic vibration imparting conditions of passes of flowNumber of layer forming Imparting type ultrasonic times of Filter holecomposition time Frequency Intensity dispersing device filteringdiameter Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 1 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 2 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 3 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 4 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 5 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 6 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 7 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 8 Comparative 4.0 parts None None None 2 times 1 time 1.0 μmExample 9 Comparative 15.0 parts  0.5 seconds 30 kHz 100 W 2 times 1time 1.0 μm Example 10 Comparative  0 part 0.5 seconds 30 kHz 100 W 2times 1 time 1.0 μm Example 11 Comparative 4.0 parts 0.5 seconds 30 kHz100 W 1 time  1 time 1.0 μm Example 12 Comparative 4.0 parts 0.5 seconds30 kHz 100 W 2 times 1 time 1.0 μm Example 13 Example 1 4.0 parts 0.5seconds 30 kHz 100 W 2 times 1 time 1.0 μm Example 2 4.0 parts 0.5seconds 30 kHz 100 W 2 times 1 time 1.0 μm Example 3 4.0 parts 0.5seconds 30 kHz 100 W 2 times 1 time 1.0 μm Example 4 4.0 parts 0.5seconds 30 kHz 100 W 2 times 1 time 1.0 μm Example 5 4.0 parts 0.5seconds 30 kHz 100 W 2 times 1 time 1.0 μm Example 6 4.0 parts 3.0seconds 30 kHz 100 W 2 times 1 time 1.0 μm Example 7 4.0 parts 0.5seconds 30 kHz 100 W 30 times   5 times 0.5 μm Example 8 4.0 parts 3.0seconds 30 kHz 100 W 30 times   5 times 0.5 μm Center line Number ofaverage surface times of roughness Ra occurrence Rate of measured ofdecrease in decrease in regarding resistance resistance surface of ServoSNR value value magnetic layer S_(after)-S_(before) FWHM_(before)FWHM_(after) ΔSFD head (dB) (times) (%) Comparative Example 1 2.8 nm 3.2nm 8.5 nm 6.9 nm 0.63 GMR 0 0 0 Comparative Example 2 2.5 nm 3.2 nm 8.5nm 6.9 nm 0.63 GMR 2.2 0 0 Comparative Example 3 2.0 nm 3.2 nm 8.5 nm6.9 nm 0.63 GMR 4.5 0 0 Comparative Example 4 1.5 nm 3.2 nm 8.5 nm 6.9nm 0.63 GMR 6.8 0 0 Comparative Example 5 2.8 nm 3.2 nm 8.5 nm 6.9 nm0.63 TMR 0.7 1 — Comparative Example 6 2.5 nm 3.2 nm 8.5 nm 6.9 nm 0.63TMR 3.2 3 — Comparative Example 7 2.0 nm 3.2 nm 8.5 nm 6.9 nm 0.63 TMR5.5 7 — Comparative Example 8 1.5 nm 3.2 nm 8.5 nm 6.9 nm 0.63 TMR 7.7 9— Comparative Example 9 2.0 nm 3.2 nm 8.5 nm 6.9 nm 0.48 TMR 7.0 7 —Comparative Example 10 2.0 nm 11.0 nm  6.8 nm 6.9 nm 0.48 TMR 7.0 — —Comparative Example 11 2.0 nm   0 nm 6.8 nm 6.9 nm 0.48 TMR 7.0 — —Comparative Example 12 2.0 nm 3.2 nm 6.8 nm 7.5 nm 0.48 TMR 7.0 7 —Comparative Example 13 2.0 nm 3.2 nm 6.8 nm 6.9 nm 0.63 TMR 5.5 0 5Example 1 2.0 nm 3.2 nm 6.8 nm 6.9 nm 0.48 TMR 7.0 0 5 Example 2 2.0 nm3.2 nm 6.8 nm 6.9 nm 0.33 TMR 7.2 0 5 Example 3 2.0 nm 3.2 nm 6.8 nm 6.9nm 0.21 TMR 7.5 0 5 Example 4 2.0 nm 3.2 nm 6.8 nm 6.9 nm 0.16 TMR 7.3 05 Example 5 2.0 nm 3.2 nm 6.8 nm 6.9 nm 0.05 TMR 7.2 0 5 Example 6 2.0nm 3.2 nm 4.1 nm 6.9 nm 0.21 TMR 7.5 0 4 Example 7 2.0 nm 3.2 nm 6.8 nm4.0 nm 0.21 TMR 7.5 0 2 Example 8 1.5 nm 3.2 nm 4.1 nm 4.0 nm 0.21 TMR9.3 0 11 

As shown in Table 1, in Examples 1 to 8, the servo pattern could be readat a high SNR by using the TMR head as the servo head. In Examples 1 to8, a significant decrease in resistance value of the TMR head could beprevented.

The invention is effective for usage of magnetic recording for whichhigh-sensitivity reproducing of information recorded with high densityis desired.

What is claimed is:
 1. A magnetic tape device comprising: a magnetictape; and a servo head, wherein the servo head is a magnetic headincluding a tunnel magnetoresistance effect type element as a servopattern reading element, the magnetic tape includes a non-magneticsupport, and a magnetic layer including ferromagnetic powder, a bindingagent, and fatty acid ester on the non-magnetic support, the magneticlayer includes a servo pattern, a center line average surface roughnessRa measured regarding a surface of the magnetic layer is equal to orsmaller than 2.0 nm, a full width at half maximum of spacingdistribution measured by optical interferometry regarding the surface ofthe magnetic layer before performing a vacuum heating with respect tothe magnetic tape is greater than 0 nm and equal to or smaller than 7.0nm, a full width at half maximum of spacing distribution measured byoptical interferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape isgreater than 0 nm and equal to or smaller than 7.0 a differenceS_(after)−S_(before) between a spacing S_(after) measured by opticalinterferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape and aspacing S_(before) measured by optical interferometry regarding thesurface of the magnetic layer before performing the vacuum heating withrespect to the magnetic tape is greater than 0 nm and equal to orsmaller than 8.0 nm, and ΔSFD in a longitudinal direction of themagnetic tape calculated by Expression 1 is equal to or smaller than0.50,ΔSFD=SFD_(25° C.)−SFD_(−190° C.)   Expression 1 in Expression 1, theSFD_(25° C.) is a switching field distribution SFD measured in alongitudinal direction of the magnetic tape at a temperature of 25° C.,and the SFD_(−190° C.) is a switching field distribution SFD measured ina longitudinal direction of the magnetic tape at a temperature of −190°C.
 2. The magnetic tape device according to claim 1, wherein the fullwidth at half maximum of spacing distribution measured by opticalinterferometry regarding the surface of the magnetic layer beforeperforming the vacuum heating with respect to the magnetic tape is 3.0nm to 7.0 nm.
 3. The magnetic tape device according to claim 1, whereinthe full width at half maximum of spacing distribution measured byoptical interferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape is 3.0nm to 7.0 nm.
 4. The magnetic tape device according to claim 1, whereinthe difference S_(after)−S_(before) is 2.0 nm to 8.0 nm.
 5. The magnetictape device according to claim 1, wherein the center line averagesurface roughness Ra measured regarding the surface of the magneticlayer is 1.2 nm to 2.0 nm.
 6. The magnetic tape device according toclaim 1, wherein the ΔSFD is 0.03 to 0.50.
 7. The magnetic tape deviceaccording to claim 1, wherein the magnetic tape includes a non-magneticlayer including non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.
 8. A head tracking servomethod comprising: reading a servo pattern of a magnetic layer of amagnetic tape by a servo head in a magnetic tape device, wherein theservo head is a magnetic head including a tunnel magnetoresistanceeffect type element as a servo pattern reading element, the magnetictape includes a non-magnetic support, and a magnetic layer includingferromagnetic powder, a binding agent, and fatty acid ester on thenon-magnetic support, the magnetic layer includes the servo pattern, acenter line average surface roughness Ra measured regarding a surface ofthe magnetic layer is equal to or smaller than 2.0 nm, a full width athalf maximum of spacing distribution measured by optical interferometryregarding the surface of the magnetic layer before performing a vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 7.0 nm, a full width at half maximum of spacingdistribution measured by optical interferometry regarding the surface ofthe magnetic layer after performing the vacuum heating with respect tothe magnetic tape is greater than 0 nm and equal to or smaller than 7.0nm, a difference S_(after)−S_(before) between a spacing S_(after)measured by optical interferometry regarding the surface of the magneticlayer after performing the vacuum heating with respect to the magnetictape and a spacing S_(before) measured by optical interferometryregarding the surface of the magnetic layer before performing the vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 8.0 nm, and ΔSFD in a longitudinal direction of themagnetic tape calculated by Expression 1 is equal to or smaller than0.50,ΔSFD=SFD_(25° C.)−SFD_(−190° C.)   Expression 1 in Expression 1, theSFD_(25° C.) is a switching field distribution SFD measured in alongitudinal direction of the magnetic tape at a temperature of 25° C.,and the SFD_(−190° C.) is a switching field distribution SFD measured ina longitudinal direction of the magnetic tape at a temperature of −190°C.
 9. The head tracking servo method according to claim 8, wherein thefull width at half maximum of spacing distribution measured by opticalinterferometry regarding the surface of the magnetic layer beforeperforming the vacuum heating with respect to the magnetic tape is 3.0nm to 7.0 nm.
 10. The head tracking servo method according to claim 8,wherein the full width at half maximum of spacing distribution measuredby optical interferometry regarding the surface of the magnetic layerafter performing the vacuum heating with respect to the magnetic tape is3.0 nm to 7.0 nm.
 11. The head tracking servo method according to claim8, wherein the difference S_(after)−S_(before) is 2.0 nm to 8.0 nm. 12.The head tracking servo method according to claim 8, wherein the centerline average surface roughness Ra measured regarding the surface of themagnetic layer is 1.2 nm to 2.0 nm.
 13. The head tracking servo methodaccording to claim 8, wherein the ΔSFD is 0.03 to 0.50.
 14. The headtracking servo method according to claim 8, wherein the magnetic tapeincludes a non-magnetic layer including non-magnetic powder and abinding agent between the non-magnetic support and the magnetic layer.